i: ■ ' '■ '' '■' 







INDUSTRIAL CHEMISTRY 



RING A SERIES OF VOLUMES GIVING 
A COMPREHENSIVE SURVEY OF 

THE CHEMICAL. INDUSTRIES 




/ 



INDUSTRIAL CHEMISTRY 

BEING A SERIES OF VOLUMES GIVING A 
COMPREHENSIVE SURVEY OF 

THE CHEMICAL INDUSTRIES 

Edited by SAMUEL RIDEAL, D.Sc. Lond., F.I.C. 

FELLOW OF UNIVERSITY COLLEGE, LONDON 

ASSISTED BY 



JAMES A. AUDLEY, B.Sc, F.I.C. 
W. BACON, B.Sc, F.I.C, F.C.S. 

E. DE BARRY BARNETT, B.Sc, A.I.C. 
M. BARROWCLIFF, M.B.E., F.I.C. 
H. GARNER BENNETT, M.Sc. 

F. H. CARR, C.B.E., F.I.C. 

S. HOARE COLLINS, M.Sc, F.I.C 
H. C. GREENWOOD, O.B.E., D.Sc, 

F.I.C. 
JAS. KEWLEY, M.A., Cantab, F.I.C. 
R. S. MORRELL, M.A., Ph.D. 



J. R. PARTINGTON, M.A., Ph.D. 
ARTHUR E. PRATT.B.Sc, Assoc.R.S.M. 
ERIC K. RIDEAL, M.B.E., D.Sc, M.A., 

Ph.D., F.I.C. 
W. H. SIMMONS, B.Sc, F.I.C 
R. W. SINDALL, F.C.S. 
HUGH S. TAYLOR, D.Sc. 
ARMAND DE WAELE, B.Sc. 
C. M. WHITTAKER, B.Sc. 
&c, &c 



ANIMAL PROTEINS 



BY 



HUGH GARNER BENNETT, M.Sc.(Leeds) 

MEMBER OF THE SOCIETY OF LEATHER TRADES' CHEMISTS; FORMERLY 

ASSISTANT LECTURER AND DEMONSTRATOR AT THE LEATHER 

INDUSTRIES DEPARTMENT OF THE UNIVERSITY OF LEEDS 

AUTHOR OF "THE MANUFACTURE OF LEATHER" 




NEW YORK 
D. VAN NOSTRAND COMPANY 
EIGHT WARREN STREET 
1921 



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[JW 6 1322 



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2454 




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PRINTED IN GREAT BRITAIN 




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29 

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GENERAL PREFACE 

The rapid development of Applied Chemistry in recent years 
has brought about a revolution in all branches of technology. 
This growth has been accelerated during the war, and the 
British Empire has now an opportunity of increasing its 
industrial output by the application of this knowledge to the 
raw materials available in the different parts of the world. 
The subject in this series of handbooks will be treated from 
the chemical rather than the engineering standpoint. The 
industrial aspect will also be more prominent than that of 
the laboratory. Each volume will be complete in itself, and 
will give a general survey of the industry, showing how 
chemical principles have been applied and have affected 
manufacture. The influence of new inventions on the 
development of the industry will be shown, as also the 
effect of industrial requirements in stimulating invention. 
Historical notes will be a feature in dealing with the 
different branches of the subject, but they will be kept 
within moderate limits. Present tendencies and possible 
future developments will have attention, and some space 
will be devoted to a comparison of industrial methods and 
progress in the chief producing countries. There will be a 
general bibliography, and also a select bibliography to follow 
each section. Statistical information will only be introduced 
in so far as it serves to illustrate the line of argument. 

Each book will be divided into sections instead of 
chapters, and the sections will deal with separate branches 
of the subject in the manner of a special article or mono- 
graph. An attempt will, in fact, be made to get away from 



vi GENERAL PREFACE 

the orthodox textbook manner, not only to make the treat- 
ment original, bnt also to appeal to the very large class of 
readers already possessing good textbooks, of which there 
are quite sufficient. The books should also be found useful 
by men of affairs having no special technical knowledge, but 
who may require from time to time to refer to technical 
matters in a book of moderate compass, with references to 
the large standard works for fuller details on special points 
if required. 

To the advanced student the books should be especially 
valuable. His mind is often crammed with the hard facts 
and details of his subject which crowd out the power of 
realizing the industry as a whole. These books are intended 
to remedy such a state of affairs. While recapitulating the 
essential basic facts, they will aim at presenting the reality 
of the living industry. It has long been a drawback of our 
technical education that the college graduate, on commencing 
his industrial career, is positively handicapped by his 
academic knowledge because of his lack of information on 
current industrial conditions. A book giving a compre- 
hensive survey of the industry can be of very material 
assistance to the student as an adjunct to his ordinary text- 
books, and this is one of the chief objects of the present 
series. Those actually engaged in the industry who have 
specialized in rather narrow limits will probably find these 
books more readable than the larger textbooks when they 
wish to refresh their memories in regard to branches of the 
subject with which they are not immediately concerned. 

The volume will also serve as a guide to the standard 
literature of the subject, and prove of value to the con- 
sultant, so that, having obtained a comprehensive view of 
the whole industry, he can go at once to the proper 
authorities for more elaborate information on special points, 
and thus save a couple of days spent in hunting through the 
libraries of scientific societies. 

As far as this country is concerned, it is believed that 
the general scheme of this series of handbooks is unique, 
and it is confidently hoped that it will supply mental 



GENERAL PREFACE vii 

munitions for the coming industrial war. I have been 
fortunate in securing writers for the different volumes who 
are specially connected with the several departments of 
Industrial Chemistry, and trust that the whole series will 
contribute to the further development of applied chemistry 
throughout the Empire. 

SAMUEI/ RIDEAI,. 



AUTHOR'S PREFACE 

It has been the author's chief concern that this volume 
should fulfil its own part in the programme set forth in 
Dr. Rideal's General Preface. 

The leather, glue, and kindred trades have been for 
many years recognized as chemical industries, but the great 
development of colloid chemistry in the last few years has 
given these trades a more definite status as such, and they 
can now be placed in the category of applied physical 
chemistry. The time is probably not far distant when 
some knowledge of pure physical chemistry will be a first 
essential to students, chemists, chemical engineers, and to 
all engaged in these industries in supervision, administration, 
or control. It is hoped that this volume will stimulate the 
study of these industries from that standpoint. 

As the author has previously written upon one of the 
industries involved herein (" The Manufacture of Leather " : 
Constable & Co.), he has, rather inevitably, found it difficult 
to avoid altogether his own phraseology. The changes of 
a decade, however, together with the wider field and newer 
view-point, have made possible a radical difference of 

treatment. 

The author desires to acknowledge the help he has 
received from the many books, essays, and researches which 
are mentioned in the references at the end of each section, 
especially to Procter's " Principles of Leather Manufacture," 
and also to thank Dr. Rideal for many useful suggestions. 
The author would like also to acknowledge here his indebted- 
ness (as well as that of the trade generally) to the work of 



x AUTHOR'S PREFACE 

Dr. J. Gordon Parker, who, through his researches, lectures, 
and teaching work, has done more than any other man to 
disseminate a knowledge of practical methods of tanning. 

The author's thanks are also due to his brother, Mr. W. 
Gordon Bennett, M.Sc, A.I.C., M.C., for assistance in proof 
revision, and to his father, Rev. John Bennett, for some 
literary criticism. 

H. GARNER BENNETT. 

Beverley, 

June, 1 92 1. 



CONTENTS 

PAGE 

GENERAL PREFACE v 

AUTHOR'S PREFACE ^T ix 

CONTENTS ,. xi 

INTRODUCTION I 



PART I. 
HIDES FOR HEAVY LEATHERS. 

SECTION 

i. The Raw Material of Heavy Leathers ... 7 

2. The Preparation of Pelt 16 

3. Vegetable Tannage 27 

4. Finishing Processes 49 

5. Sole Leather .55 

6. Belting Leather . . . . . . . .65 

7. Harness Leather 7I 

8. Upper Leathers : . . .76 

9. Bag Leather 86 

10. Picking Band Butts 9 o 



PART II. 

SKINS FOR LIGHT LEATHERS. 

1. Principles and General Methods of Light Leather 

Manufacture 92 



xii CONTENTS 

SECTION PAGE 

2. Goatskins ... 99 

3. Sealskins 106 

4. Sheepskins . . .110 

5. Calfskins 120 

6. Japanned and Enamelled Leathers . . . .123 



PART III. 
CHROME LEATHERS. 

1. The Nature of Chrome Leathers 127 

2. General Methods of Chrome Leather Manufacture 139 

3. Chrome Calf .156 

4. Chrome Goat and Sheep 163 

5. Heavy Chrome Leathers 170 



PART IV. 
MISCELLANEOUS TANNAGES. 

1. Alum Tannages 174 

2. Fat Tannages 178 

3. Oil Tannages 181 

4. Formaldehyde Tannage 185 

5. Synthetic Tanning Materials 187 

6. Combination Tannages I9 i 

7. The Evolution of the Leather Industry . . .194 



PART V. 

GELATINE AND GLUE. 

1. Properties of Gelatine and Glue 200 

2. Raw Materials and Preliminary Treatment . . 220 



CONTENTS xiii 

SECTION PAGE 

3. Extraction . 230 

4. Clarification and Decolorization 234 

5. Bleaching 241 

6. Evaporation 248 

7. Cooling and Drying . • 255 

8. Uses of Gelatine and Glue 260 

9. The Evolution of the Gelatine and Glue Industry 265 



PART VI. 
MISCELLANEOUS PROTEINS AND BYE-PRODUCTS. 

1. Bye-products of the Leather Trades .... 268 

2. Bye-products of the Gelatine and Glue Trades . 272 

3. Food Proteins 274 

4. Miscellaneous Animal Proteins 279 

INDEX 283 



ANIMAL PROTEINS 



INTRODUCTION 



Proteins are organic compounds of natural origin, being 
found in plants and in animals, though much more plenti- 
fully in the latter. They are compounds of great complexity 
of composition, and of very high molecular weight. The 
constitution of none of them is fully understood, but although 
there are a great number of different individual proteids, 
they present typical resemblances and divergences which 
serve to differentiate them from other groups of organic 
bodies, and also from one another. 

Proteins resemble one another in both proximate and 
ultimate analysis. They contain the usual elements in 
organic compounds, but in proportions which do not vary 
over very wide limits. This range of variation is given 
approximately below : — 



Element. 


Per cent. 


Carbon 


.. 49 to 55 


Hydrogen 


6-4 to 7-3 


Oxygen 


., 17 to 26 


Nitrogen 


13 to 19 


Sulphur 


03 to 30 



The most characteristic feature of the protein group is 
the amount of nitrogen usually present. This is generally 
nearer the higher limit, seldom falling below 15 per cent. 
This range for the nitrogen content is determined largely 
by the nature of constituent groups which go to form the 
proteid molecule. Roughly speaking, proteins consist of 
chains of amido-acids and acid amides with smaller pro- 
portions of aromatic groups, carbohydrate groups and thio 
E. 1 



2 ANIMAL PROTEINS 

compounds attached. In these chains an acid radical may 
combine with the amido group of another amido acid, the 
acid group of the latter combining with an amido group of 
another amido acid, and so on. Hydrogen may be substituted 
in these chains by alkyl or aromatic groups. There is 
obviously infinite possibility of variation in constitution 
for compounds of this character, the general nature of which 
varies very little. Practically all of the proteins are found 
in the colloid state, and this makes them very difficult to 
purify and renders the ultimate analysis in many cases 
doubtful. It is, for example, often difficult to ascertain 
their moisture content, for many are easily hydrolyzed with 
water only, and many part easily with the elements of water, 
whilst on the other hand many are lyophile colloids and 
practically cannot be dehydrated or dried. A few, such 
as gelatin and some albumins, have been crystallized. 

The constituent groups have been investigated chiefly 
by hydrolytic methods. The chains of amido acids are 
split up during hydrolysis, and individual amido acids may 
thus be separated. The hydrolysis may be assisted either 
by acids, alkalies or ferments, but follows a different course 
according to the nature of the assistant. Under approxi- 
mately constant conditions of hydrolysis, the products 
obtained are in approximately constant proportions, and 
this fact has been utilized by Van Slyke in devising a method 
of proximate analysis. It is not possible in this volume to 
enter deeply into the constitution of the different proteids. 
Reference must be made to works on pure chemistry, 
especially to those on advanced organic chemistry. It will 
be interesting, however, to mention some of the amido 
acids and groups commonly occurring in proteids. These 
comprise ornithine (i : 4 diamido valeric acid), lysine (1 : 5 
diamido-caproic acid), arginine (1 amido, 4 guanidine valeric 
acid), histidine, glycine (amidoacetic acid), alanine (amido 
propionic acid), amido- valeric acid (amido-iso-caproic acid), 
liacine, pyrollidine carboxylic acid, aspartic acid, glutamic 
acid (amido-glutaric acid), phenyl-alanine, serine (hydroxj*- 
amido propionic acid), purine derivatives {e.g. guanine), 



INTRODUCTION 3 

indol derivatives {e.g. tryptophane and skatol acetic acid), 
cystine (a thioserine anhydride), glucosamine, and urea. 

There are a few general reactions which are typical of all 
proteins, and which can usually be traced to definite groupings 
in the molecule. Amongst these is the biuret reaction : a 
pink colour obtained by adding a trace of copper sulphate 
and an excess of caustic soda. This is caused by the biuret, 
NH(CO.NH 2 )2 radical or by similar diacidamide groups, 
e.g. malonamide, oxamide, glycine amide. Another general 
reaction is with " Millon's reagent," a solution of mercuric 
nitrate containing nitrous fumes. On warming the proteid 
with this reagent, a curdy pink precipitate or a red colour 
is obtained. This reaction is caused by the tj-rosine group 
(p. oxy a amido phenyl-propionic acid). Another general 
reaction is to boil the protein with 1 : 2 nitric acid for some 
days. A yellow flocculent precipitate of " xanthoproteic 
acid " is obtained, and this dissolves in ammonia and caustic 
alkalies with a brown or orange-red colour. Another 
characteristic of proteins is that on dry distillation they )deld 
mixtures of pyridine C 5 H 5 N, pyrrol C 4 H 5 N, and their 
derivatives. 

On the subdivision, classification and nomenclature of 
the proteins much ink has been spilled, and it is impossible 
in this volume to go into the various systems which have 
been suggested. It should be noted, however, that some 
writers habitually use the terms " proteid " or " albuminoid " 
as synonyms for protein. The classification of proteins 
adopted in this work is used because it is the most suitable 
for a volume on industrial chemistry and has the additional 
merits that it is simple and is already used in several 
standard works on industrial chemistry. It is based upon 
the behaviour of the proteins towards water, a matter of 
obvious moment in manufacturing processes. On this basis 
proteins may be divided into albumins, keratins and gelatins. 
Cold water dissolves the albumins, does not affect the 
keratins, and only swells the gelatins. The behaviour in 
hot water confirms and elaborates the classification. When 
heated in water, the albumins coagulate at temperatures of 



4 ANIMAL PROTEINS 

7o°-75° C, the gelatins (if swollen) dissolve readily, whilst 
the keratins only dissolve at temperatures above ioo° C. 
Albumins and keratins may be distinguished also from 
gelatins by adding acetic acid and potassium ferrocyanide 
to their aqueous solutions. Albumins and keratins give a 
precipitate, gelatins do not. Another distinguishing reaction 
is to boil with alcohol, wash with ether, and heat with hydro- 
chloric acid (S.G. 1/2). Albumins give a violet colour, 
keratins and gelatins do not. 

Albumins may be first discussed. They are typified 
by the casein of milk and by white of egg. Their solutions 
in water are faintly alkaline, optically active, and laevo- 
rotatory. They are coagulated by heat and also by mineral 
acids, alcohol, and by many poisons. The temperature of 
coagulation (usually about 72 C.) is affected by mineral 
salts, the effect being in lyotrope order (see Part V., Section 
I.). The coagulated albumin behaves in most respects like 
a keratin. Some of the albumins (globulins) are, strictly 
speaking, not soluble in cold water, but readily dissolve in 
weak solutions of salt. The albumins are coagulated from 
these solutions, as usual, when heated. Into this special 
class fall myosin (of the muscles), fibrinogen (of the blood) 
and vitellin (of egg yolk) . By a gentle or limited hydrolysis 
of the albumins with dilute acids in the cold, a group of 
compounds called albuminates are obtained. They dissolve 
in either acids or alkalies, and are precipitated by exact 
neutralization. They may also be " salted " out by adding 
sodium chloride or magnesium sulphate. They are not 
coagulated by heat. After further hydrolysis with either 
acids, alkalies or ferments, very soluble compounds are 
obtained called albumin peptones or albumoses. These are 
soluble in alkalies, acids and water, and are readily hydro- 
lyzed further into amido acids and acid amides. They are 
very similar to the peptones obtained from keratins and 
gelatins. They are not coagulated by heat. 

Keratins are typified by the hair of animals. They 
soften somewhat in cold water and even more in hot water, 
but are not dissolved until digested for some time at tern- 



INTRODUCTION 5 

peratures exceeding ioo° C. With some keratins, however, 
the cystine group is to some extent easily split off by warm 
water, and on boiling with water hydrogen sulphide is 
evolved. The sulphur content of keratins is often greater 
than the average for proteids. All keratins are dissolved 
with great readiness by solutions containing sulphydrates 
and hydrates, e.g. a solution of sodium sulphide. In solu- 
tions of the hydrates of the alkali and alkaline earth metals, 
keratins behave differently. Some dissolve with great ease, 
some with difficulty, some only on heating and some not 
even if digested with hot caustic soda. They are dissolved 
(with hydrolysis) by heating with mineral acids, yielding 
peptones and eventually amido acids, acid amides, etc. 
Many keratins have a comparatively low content of nitrogen. 
Gelatins are very difficult to distinguish from one 
another, their behaviour being closely similar to reagents. 
They are also very readily hydrolyzed even with water, and 
the products of hydrolysis are even more similar. The 
gelatins are known together, commercially, under the 
general name of gelatine. Gelatins of different origin, how- 
ever, have undoubtedly a different composition, the nitrogen 
content being variable. If the gelatins are not bleached 
whilst they are being manufactured into commercial gelatine, 
they are called "glue." Gelatine is colourless, transparent, 
devoid of taste and smell. It is usually brittle. Its S.G. is 
about 1-42, and it melts at 140 C. and decomposes. It is 
insoluble in organic solvents. When swelling in cold water 
it may absorb up to 12 times its own weight of water. The 
swollen product is called a " jelly." Jellies easily melt on 
heating and a colloidal solution of gelatine is obtained. 
This " sets " again to a jelly on cooling, even if only 1 per 
cent, gelatin (or less) be present. The solution is optically 
active and lsevorotatory, but with very variable specific 
rotation. Some observers have thought that the different 
gelatins have different specific rotations and may so be 
distinguished. Gelatins are precipitated from solutions by 
many reagents, such as alcohol, formalin, quinone, meta- 
phosphoric acid, tannins, and many salt solutions, e.g. those 



6 ANIMAL PROTEINS 

of aluminium, chromium and iron, and of mercuric chloride, 
zinc sulphate, ammonium sulphate, potassium carbonate, 
acidified brine. Many of these precipitations have analogies 
in leather manufacture (see Parts I. to IV.). The gelatin 
peptones or gelatoses are formed by hydrolysis with acids, 
alkalies, ferment or even by digestion with hot water only . 
A more detailed description of the properties of gelatine is 
given in Part V., Section I. Gelatine is sometimes called 
" glutin " and " ossein." 

Animals are much the most important source of proteins, 
especially of those which are of importance in industrial 
chemistry. Proteins occur in nearly every part of all 
^imals, and the "protoplasm" of the living cell is itself 
. ^eotein. The keratins include the horny tissues of 
animals : the epidermis proper, the hair, horns, hoofs, 
nails, claws, the sebaceous and sudoriferous glands and ducts, 
and also the elastic fibres. The gelatins are obtained from 
the collagen of the skin fibres, the bones, tendons, ligaments, 
cartilages, etc. Fish bladders yield a strong gelatin. The 
albumins are obtained from the ova, blood, lymph, muscles 
and other internal organs of animals. 

The classification of proteins herein adopted fits in well 
with the scope and purpose of this volume. The keratins 
are of little importance in chemical industry, but are of 
immense importance in mechanical industry, e.g. the woollen 
trade, which is based upon the keratin comprised by sheep 
wool. The collagen of the hide and skin fibres is of vast 
importance to chemical industry, and is the basis of the 
extensive leather trades discussed in Parts I. to IV. The 
waste pieces of these trades, together with bones, form the 
raw material of the manufacture of gelatin and glue, as 
discussed in Part V. The proteids of animals' flesh and 
blood, milk and eggs form the source of the food proteins 
discussed in Part VI. The food proteins embrace chiefly 
albumins, but gelatins and even keratins are involved to 
some extent. 



Part I.— HIDES FOR HEAVY LEATHERS 

Section L— THE RAW MATERIAL OF 
HEAVY LEATHERS 

The term " hide " possesses several shades of meaning. 
In its widest sense it applies to the external covering of 
all animals, and is sometimes used derogatively for hump- 
skin. In this wide sense, it is almost synonymous * 
the term " skin." The term " hide," however, hab a 
narrower meaning, in which it applies only to the outer 
covering of the larger animals, and in this sense is used 
rather in contrast with the term " skin." Thus we speak 
of horse hides, cow hides, camel hides, and buffalo hides. 
It is used in this sense in the title of Part I. of this volume. 
As such hides are from large animals, the leather which is 
manufactured therefrom is thick and in large pieces, and 
is therefore commercially designated as "heavy leather." 
From the standpoint of chemical industry hides are amongst 
the most important of animal proteins, and their trans- 
formation into leather for boots, shoes, belting, straps, 
harness, and bags comprises the " heavy leather trade," 
which is one of the largest and most vital industries of the 
country. The heavy leather trade predominates over other 
branches of leather manufacture, not only because of the 
comparatively large weight and value of the material 
handled, but also because the resulting products have a 
more essential utility. There is also a still narrower use 
of the term " hide," in which it applies only to the do- 
mesticated cattle — the ox, heifer, bull and cow — which 
use arises from the fact that the hides of these are both 
the largest and most valuable portion of the raw material 



8 ANIMAL PROTEINS 

of the heavy leather industries. In a very narrow sense 
the term is also sometimes applied only to ox hides, 
which for most heavy leathers are the ideal raw 
material. 

The Home Supply of hides forms a large important 
proportion of the total raw material. Its importance, 
moreover, is rapidly increasing, for the excellence and 
abundance of the home supply determines the extent to 
which it is necessary for the industry to purchase its raw 
material abroad. The position of our national finances 
makes this an increasingly serious matter, for hides are 
comparatively a very expensive material. 

The quality of our home supply of hides is very valu- 
able, being determined by the conditions of the animal's 
life, its precise breed, and by other factors such as age 
and sex. The best hides are usually obtained from animals 
which have been most exposed to extremes of wind and 
cold, as such conditions tend naturally to develop a thicker 
and more compact covering. Broadly speaking, these 
include the hides from cattle of the northern and hilly 
districts. The age of the animal when killed is also a 
dominating factor. Calf skins are very soft, fine grained 
and compact, the state of rapid growth favouring the 
existence of much interfibrillar substance. The youngest 
animals supply suitable raw material for various light 
leathers (see Part II., Section V., p. 120), and are also very 
suitable for chrome work (see Part III., Section III., p. 156). 
Bull and cow hides, on the other hand, are from animals 
whose growth is complete, and show in consequence a lack 
of interfibrillar substance, coarse fibres and a rough and 
often wrinkled grain. The resulting leather tends conse- 
quently to be spongy, thin, empty and non- waterproof. 
Intermediate between these extremes are the hides of the 
ox and heifer, large, yet of good texture, and well supplied 
with interfibrillar substance. These hides are much the 
best for sole leather, a firm, smooth-grained and well-filled 
leather being needed. The term " kip " is often applied 
to small hides and to hides from large calves. In the 



THE RAW MATERIAL OF HEAVY LEATHERS 9 

trade, however, "kip" is sometimes used also for larger 
hides, as a verbal enhancement of value ; just as a man 
with a few old fowls is said to keep " chickens." Cow hides 
tend to be " spready," i.e. to have a large area per unit 
weight, and are therefore more suitable for dressing leather. 
Bull hides are thicker in the neck and belly, and thinner 
in the back, which characteristics reduce their commercial 
value. 

Market hides are sold by weight, and are therefore 
classified chiefly by their weight, Which is marked on near 
the tail by a system of knife-cuts. The animals are flayed 
after cutting the hide down the belly and on the inside 
of the legs. 

Of the various breeds, "Shorthorns" yield a large 
supply of useful hides. The name, however, covers a 
variety of similar breeds, and the hides therefrom are 
rather variable in texture and quality. They tend to be 
greasy owing to high feeding. The " Herefords," ob- 
tained from Midland markets, are generally excellent hides 
for sole and harness leathers. They give a good yield of 
butt pelt, a stout and smooth shoulder, and are not often 
greasy. " Devons " yield a good-textured and well-grown 
hide, but are often badly warbled (see p. 10). The 
"Sussex" cross breeds yield somewhat larger hides. 
" Suffolk Red Polls," common in East Anglia, yield a good 
butt, and the cow hides make good dressing leather. 
" Channel Island " cattle yield very thin hides, but with a 
fine undamaged grain. Scotch hides possess deservedly 
the very highest reputation. The climatic conditions 
favour the production of a hardy race of cattle with thick 
well-grown hides, yielding a large proportion of butt. 
These hides are amongst the best obtainable for heavy 
leather, and particularly for sole leather. " Highlanders," 
" Aberdeen Angus," " Galloways " are typical breeds, with 
short neck, legs and straight backs. Cross breeds are also 
excellent {e.g. " Scotch Shorthorns "). The natural value 
of these hides is further enhanced by the usual care in 
flaying. " Ayrshires " yield good milch cows and conse- 



io ANIMAL PROTEINS 

quently yield often a more spready hide. The Welsh breeds 
for rather similar reasons also yield valuable hides. The 
Irish " Kerrys " are small but stout, and yield hides suitable 
for light sole leather. Irish crossbreeds, Shorthorns, have 
a rather bad reputation, and are often ill flayed. 

All the varieties of the home supply are subject to 
various defects, which influence seriously their commercial 
value. One of these defects is warble holes or marks, 
caused by the Ox Warble fly (Hypoderma bovis). This is 
a two-winged fly about half an inch long. The larva of 
this fly, the " Warble maggot," lives and thrives in the skin 
of cattle, and causes a sore and swelling. The life-history 
of this insect is still in dispute, but it is generally thought 
that the eggs are laid in the hair on the animal's back, 
and the young larva eats its way through the hide until 
just below the dermis, and there feeds until mature. It 
then creeps out of this " warble hole," falls to the ground, 
pupates for a month, after which the imago or perfect 
insect emerges from the chrysalis. Hides which have been 
thus infected have, in consequence, often quite a number of 
holes through the most valuable part of the hide, thereby 
rendering it unsuitable for many kinds of leather. Even 
old "warbles" which have more or less healed up are a 
weakness, and warbled hides and leather fetch a decidedly 
lower price than undamaged. Another of these defects is 
bad flaying. Clearly the hide should be as little cut as 
possible, but many of our market hides are abominably 
gashed and often cut right through. This, of course, often 
reduces seriously the commercial value of the hide. Careless 
treatment after flaying also results in another common 
defect, viz. taint. As the term implies, the hide is partly 
putrefied, sometimes only in patches, but sometimes 
also so extensively as to render the hide quite rotten and 
quite incapable of being made into leather at all. Hides 
are of course putrescible, and dirt, blood, dung and warm 
weather encourage rapid putrefaction. As market hides 
are usually uncured, this defect is constantly appearing, 
and is a cause of considerable loss. Other defects are due 



THE RAW MATERIAL OF HEAVY LEATHERS n 

to injuries to the animal before it is killed, e.g. brands, 
scratches due to hedges and barbed wire, old scabs, goad 
and tar marks. All these reduce the value of the hide. 

All the defects in hides involve a very serious loss to 
the community, and the time is rapidly approaching when 
their continuance is insufferable. The loss is not usually 
very considerable to any individual, though very large in the 
aggregate. The hide is a minor part of the beast's value, 
and a somewhat damaged hide does not involve a very 
serious loss to the farmer. Some with typical stupidity 
regard a few warbles as "the sign of a healthy beast." 
These defects involve practically no loss to the hide 
merchant, tanner or currier, as each pays less for damaged 
material. The loss falls upon the community, and the time 
is ripe for the community to insist upon the elimination of 
these defects. The national resources will be for some 
years strained to their uttermost, and preventable damage 
must be considered intolerable. The principal defects in 
hides are preventable, and ought to be prevented. The 
warble fly could, by a united effort, be rendered before long 
practically extinct, a task which is facilitated by the fact 
that it is not migrative. Bad flaying and careless treat- 
ment of hides resulting in putrefaction are still more easily 
remedied. The communal slaughter-house is long overdue 
from the standpoint of public health, and would, under 
conditions of cleanliness and skilled workmanship and 
oversight, also solve the problem of ill-flayed and tainted 
hides. 

The question of the raw material is of first importance 
to the leather trades. There was, before the commencement 
of the European War, a steadily increasing scarcity of hides, 
causing a constant increase in their price. This was due 
partly to the fact that cattle were increasing at a less rate 
than the population, partly to the growth of civilization, 
and more extensive use of leather in proportion to the 
world's population, and partly to the constant discovery 
of new uses -for leather, e.g. for motor cars, aeronautics, etc. 
The question of raw material was under these conditions 



12 ANIMAL PROTEINS 

serious enough. The terrific slaughter, necessary at the 
same time to provide the belligerents with food and the 
army with leather, is bound to result in a serious crisis for 
the leather industries ; and in conjunction with the country's 
financial condition, will make it absolutely necessary that 
all care should be taken with the raw material of one of our 
most important industries. The farmer who pays no heed 
to the warble fly, the man who gashes the hide in flaying 
and who allows the hide to putrefy, are equally criminal 
with the man who throws bread crusts into the dustbin. 

It is impossible to foresee, as yet, anything in the nature 
of a satisfactory solution to the problem of raw material, 
especially in respect to heavy leather production, for the 
food question will rank first in the popular mind, and the 
earlier slaughter enjoined for the more economical produc- 
tion of meat will scarcely tend to increase the proportion 
of heavy hides. 

The Foreign Supply of hides is also of great importance 
and value. In the case of imported hides precautions to 
prevent putrefaction are essential, and some method of 
" curing " is always used. 

Salting the hides is one of the most satisfactory methods 
for temporary preservation. The action of salt is hygro- 
scopic, and mildly antiseptic. Moisture is withdrawn from 
the hides, which are then under conditions no longer favour- 
ing the growth of bacteria. Well-salted hides will keep for 
years, especially if quite clean. A light salting is also useful 
for a short preservation, and is becoming common in hide 
markets and tanneries during the summer and autumn 
months. Salting is a method used extensively in the 
United States. The " packer hides " of the stockyards 
are carefully and systematically salted with about 25 
per cent, of salt and stored in cool cellars. The hides are 
so piled up in heaps, that brine easily drains away. The 
great disadvantage of salting is the so-called " salt stains." 
These stains have been ascribed to the iron in the salt, to 
the iron in the blood, to calcium sulphate in the salt, and 
also to chromogenic bacteria, whose development is favoured 



THE RAW MATERIAL OF HEAVY LEATHERS 13 

by salting. The relative importance of these factors is not 
yet satisfactorily determined, but cleanliness and pure salt 
tend to eliminate the trouble. 

Drying the hides is a less satisfactory cure. The 
principle is similar, viz. removal of moisture. Dried hides 
are, however, much drier than salted, and are quite hard 
and horny, hence the name " flint hides." The hides also 
lose much weight, a considerable advantage in reducing 
freight. Tropical hides are often flint-dry, and where 
preservatives are expensive or unprocurable, it is often the 
only practicable method of cure. Nevertheless, the method 
has many serious disadvantages, and is difficult to execute. 
If dried too slowly the hides putrefy partially ; if too quickly 
they dry on the outside, and the interior is left to putrefy. 
The fact that hides are of uneven thickness,, and the climate 
often hot, increases the difficulty, and often results in 
partial destruction of the fibrous structure of the hide. 
When dried, moreover, the hides are still subject to the 
attacks of insect larvae, for the prevention of which the 
usual sprinkling of naphthalene or arsenic is only an imper- 
fect remedy. This method of cure is also a nuisance to the 
tanner, who has to employ labour, pits and time in attempt- 
ing to restore the hides to their original condition, and often 
loses up to ten per cent, of the goods in so doing. Dried 
hides are also subject to the presence of anthrax. 

Dry Salting the hides is an excellent method of curing. 
As the name implies, it combines methods of drying and 
salting which are used alternatively. The method is used 
extensively in South America. A modified form of it is 
also used for preserving the " E.I. kips," which are cured, 
however, not with common salt, but with earth containing 
up to 70 per cent, of sodium sulphate. Dry-salted hides are 
largely free from the defects of dried hides, but of course 
are more trouble to the tanner in the process of soaking 
(see Section II., p. 16) than the wet-salted goods. 

Freezing the hides is now a commercial process. On 
the whole the process is satisfactory, but the expansion of 
water after freezing may tend to damage the hide fibres. 



14 ANIMAL PROTEINS 

Sterilizing the hides has been frequently suggested, 
but no method has yet been advocated which does not 
interfere either with the tanning processes or with the 
quality of the finished leather. 

Hides from the European Continent, usually wet salted 
and well flayed, exhibit much the same variable quality 
as the home supply, those from highland districts tend- 
ing to be thick, yet even, well grown, tight textured and 
smooth grained, whilst those from lowland regions are 
less satisfactory. Thus hides from the Swiss Alps and 
Scandinavia have ranked high, whilst the spready Dutch 
cows are typical of a lowland hide. In the hides which 
once came from Germany the same features appear. Ba- 
varian highland hides had an excellent reputation, whilst 
those from Berlin, Cologne, etc., tended to be long in shank 
and not well grown. French hides are often ill flayed, and 
Spanish and Portuguese are often subject to scratches. 
Italian hides have a very good name, being small but stout 
in butt. 

The American supply is important. South America 
yields an excellent class of hide, salted or dry-salted. They 
are from an excellent breed of animals, slaughtered and 
flayed with every care, and efficiently cured. A most 
serious defect in this class of hide is the " brand," which 
is both deep and large and in the most valuable part of the 
hide. One side, however, is usually unbranded, so that each 
hide yields one good " bend." These hides, e.g. " Frigori- 
fics," have recently been much more extensively tanned in 
Britain because of the shortage in the home supply of market 
hides caused by the European War. South America also 
yields good horse hides. North American hides are usually 
wet-salted (e.g. packer hides). They are usually good. 
Central America yields mostly dried hides exhibiting the 
usual defects. 

The Asiatic supply comprises the frozen China hides, 
which are clean but small, with flaying of uncertain quality. 
There are the buffalo hides from Asia and East Europe, 
which are suitable for cheap and sole and strap leather, 



THE RAW MATERIAL OF HEAVY LEATHERS 15 

and also the dry-salted "E.I. kips," obtained from a small 
breed of Indian cattle, and extensively made into upper 
leather. The Asiatic humped cattle also provide a limited 
supply. The African supply is of increasing importance. 
The tropical parts yield dried hides of uncertain quality, 
but the more temperate parts of South Africa yield a growing 
supply of good qualit}^. 



REFERENCES. 

: The Manufacture of Leather " (Bennett), pp. 27-37. 
Principles of Leather Manufacture " (Procter), pp. 33-56. 
The Ox Warble or Bot Fly " (E. Ormerod). 
The Making of Leather " (Procter), pp. 2-22. 



Section II.— THE PREPARATION OF PELT 

Before hides are tanned it is necessary for them to pass 
through a series of preparatory processes. The object of 
these processes is to obtain from the hide the true hide 
substance in a pure and suitable condition. Each class of 
leather has its own appropriate processes, the adjustment 
of which largely determines the quality of the finished 
article. So prominent is the influence of these preparatory 
methods that the paradox " good leather is made before 
tanning " is in trade circles almost a platitude. These 
processes, sometimes lumped together under the general 
name of " Wetwork," comprise soaking, liming, beam 
house work and deliming. These will be discussed in 
turn. 

The term applied to the hide after these processes, but 
before tannage, is " pelt." 

Soaking has for its object the cleansing and softening 
of the hides, chiefly by means of water. It aims at the 
removal of dirt, blood, dung, and curing materials by 
washing. The process is usually simple, and is much the 
same for all classes of leather. The ideal to be aimed at 
is to restore the hide to its condition when it left the animal's 
back. Cleanliness in leather manufacture is as essential 
at the commencement as anywhere, for the hide is in its 
most putrescible state. The soluble proteids (blood, lymph, 
part of dung, etc.) which always adhere to hides en- 
courage the rapid growth of putrefactive bacteria, and 
cannot be washed away too soon. Dung is often difficult 
to remove, being caked on the butt end amongst the hair. 
Soaking only softens it, and mechanical removal is usually 
necessary. If such substances are not removed, they go 



THE PREPARATION OF PELT 17 

forward with the goods into the lime liquors, causing stains, 
loss of hide substance, and counteracting plumping. 

The detailed method and time of soaking are determined 
mainly by the nature of the cure. One of the purposes of 
the soak liquors is to dissolve the salt used in curing hides 
and to rehydrate the hide and make it again soft and pliable. 
As a 10-per-cent. salt solution exerts a solvent effect on hide 
substance, it is necessary soon to change the first soak liquor 
of salted goods. 

Market hides, which are uncured, require the least 
soaking, the cleansing effect being most needed. The 
hides are inserted into pits (" water dykes ") of water for 
a few hours, and the water changed once or twice. The 
soaking should not be prolonged as the hides are so putre- 
scible, and where it is customary to leave the goods in a 
soak liquor overnight, it is advantageous to add a little slaked 
lime to the water before inserting the goods. This not only 
softens hard water, but is mildly antiseptic and plumping, 
and forms a suitable introduction to the liming proper. 
Each pit contains a " pack " of 30-50 hides, according to 
its capacity, which varies in different tanneries from 1000 
to 2000 gallons. Tainted goods, which are indicated by 
a characteristic white colour on the flesh side and by loose 
hair, need a preliminary washing either in a drum," 
"tumbler" or in a "paddle." This ensures a rapid 
change of liquor and the removal of most of the putre- 
factive agencies. Bad cases may need the application of 
antiseptics, such as immersion in o-i per cent, carbolic 
acid ; but if possible these should be avoided, as they 
lengthen the time required for liming. After drumming 
or paddling, tainted goods should be placed directly into a 
lime liquor. 

Salted hides need very similar treatment to uncured 
hides, but the soaking is longer, because of the dehydration 
caused by salting. Hence they receive also a greater number 
of changes of water, three or four usually, but often more. 
As much loose salt as possible should be shaken from the 
hides before insertion into any liquor. The employment of 

E. 2 



18 ANIMAL PROTEINS 

drum or paddle before pit soaking is extremely useful to 
effect the rapid removal of superficial salt, and is also 
useful after pit soaking to remove the last traces. 

Dried and dry-salted goods need a soaking still more 
prolonged, up to one week if water alone be used. With 
the assistance of caustic soda, however, the process can be 
shortened to about two days. The first soak liquor should 
consist of a o-i per cent, solution of caustic soda, and after 
the goods have been inserted twenty-four hours, they will 
be materially improved by a few hours' drumming or 
paddling. Another caustic soda soak will complete the 
process. Sodium sulphide crystals may replace caustic 
soda, but about three times the weight will be needed. 
Carbonate of soda and caustic lime also are a convenient 
commercial substitute for caustic soda. For 10 lbs. caustic 
soda, use 36 lbs. carbonate and 7 lbs. lime. Extra lime 
should be added in all cases when the water is hard. Acid 
liquors will also soften dried and dry-salted goods, but 
such processes do not fit in so well with the subsequent 
liming. The use of putrid soaks and stocks may be now 
considered out of date. 

Liming follows soaking, and consists essentially in 
immersing the hides for 7-10 days in milk of lime. The 
chief object in view is to loosen the hair and prepare for 
its mechanical removal. Liming takes place in pits, the 
tops of which are level with the limey ard floor. The lime 
is slaked completely and mixed well with water in the pit, 
being particularly well plunged just before the insertion 
of a pack of goods. Saturated limewater is only a 0-13- 
per-cent. solution. The goods are occasionally " handled," 
i.e. hauled out of the pit and reinserted after plunging 
("hauling" and "setting"). This is necessary to keep 
the liquor saturated with lime. The hides are inserted 
one by one, each being " poked down " to ensure its contact 
with the liquor. The goods are invariably immersed first 
in a previously used lime liquor. Most tanneries now carry 
this out in a systematic way, so as to ensure regularity in 
the process. As the goods are large and heavy it is less 



THE PREPARATION OF PELT 19 

laborious to carry out the whole process in one pit. In this 
" one-pit system " the goods are inserted for (say) four 
days in an old used lime liquor, with occasional handling ; 
this liquor is then run to the drain and a new liquor made 
up in the same pit, into which the goods are inserted for 
(say) five days. They are then hauled and sent to the 
unhairers. Each pack thus gets two liquors, old and new. 

A better method is the " three -pit system." In this 
case each pack receives three liquors and has (say) three 
days in each, first an " old lime," then a " medium lime," and 
finally a "new lime." This system ensures a greater regu- 
larity of treatment, and is deservedly the most popular 
method for liming hides for sole leather. After being used 
once as a " new lime," a liquor then becomes a " medium 
lime," and after being thus used becomes the " old lime " 
which receives the green hides from the soaks. The 
system involves the goods being shifted twice to another 
pit, which is more laborious than reinsertion into the old 
pit, but if the limey ard be arranged in " sets " or " rounds " 
of three pits, the shift is usually only to the adjacent pit. 
One special advantage of this system is that the top hides 
in one pit become the bottom hides in the next pit, and vice 
versa. Rounds of more than three pits are sometimes used. 

Many factories have now adopted systems in which 
there is no handling at all. The hides are suspended in lime 
liquors which are agitated by mechanical contrivances (e.g. 
Tilston-Melbourne process), or by jets of compressed air 
(e.g. Forsare process). The goods are soaked and limed 
"mellow to fresh" by changing the liquors by means of 
pumps, air ejectors, etc. Thus the hides need no labour 
from first being inserted until drawn for depilation. 

In liming, the whole of the epidermis as well as the 
hair is loosened, and is subsequently removed in depilation. 
The corium or true hide substance becomes much more 
swollen by imbibation of water, and when taken out of 
the new lime is " plumped " to very firm jelly. This 
plumping is a matter of prime importance to the tanner. 
The coarser fibres are thereby split up into the finer 



20 ANIMAL PROTEINS 

constituent fibrils, which fact assists very materially in obtain- 
ing a quick and complete tannage, good weight, and a firm 
leather. During the liming, the natural grease of the hide 
is saponified or emulsified, which prepares for its removal 
in scudding. Liming is thus a complex process : the hair is 
loosened, the hide is plumped, and the grease is " killed." 
All these results may be hastened by the use of other 
alkalies in addition, and most heavy leather yards assist 
the liming by adding also sodium sulphide or caustic soda 
or both. Sodium sulphide is a powerful depilatant, and will 
alone unhair hides easily in strong solutions even in a few 
hours. As in solution it forms caustic soda by hydrolysis, 
it possesses also the powerful plumping and saponifying 
powers characteristic of the latter. The addition of arsenic 
sulphide (As 2 S 2 ) (realgar) to the lime when slaking causes 
the presence of calcium sulphydrate in the lime liquors thus 
made. This is also a powerful depilatant, but not much 
used for heavy leather. 

The function of the lime in depilating is complex and 
has occasioned much discussion. Its main purpose, how- 
ever, is that of a partial antiseptic. When hides putrefy, 
one of the first results is that the hair is loosened. In 
America depilation by " sweating " is carried out com- 
mercially by such a mild putrefaction, trie lime liquor 
permits a similar fermentation at a slower rate, and all 
tannery lime liquors are swarming with putrefactive bacteria. 
Liming is thus a safer method than sweating, which may be 
easily carried too far. Various workers have isolated 
specific organisms — Wood a bacillus, Schmitz-Dumont a 
streptococcus — but it seems highly probable that the 
limeyard bacteria are just the common organisms of putre- 
faction sorted out or selected by the exact nature of the 
liquor and the method of working the limes. Many putre- 
factive bacteria are very adaptable and could easily ac- 
commodate themselves in this way. It is known that the 
exact nature of the culture-medium has a great influence 
on the rate of development of such organisms, and which 
particular species thrive and obtain predominance in any 



THE PREPARATION OF PELT 21 

limeyaid will depend upon the amount and nature of the 
dissolved organic matter available as food, and upon the 
exact alkalinity and the concentration of other apparently 
inert subtances, such as common salt and sodium, calcium 
and arsenic salts. Hence no two lime liquors operate alike, 
and approximate regularity is only assured by systematic 
method. In handling and shifting, the organisms are 
subjected to further selection, and the most adaptable 
survive. It is probable that different species may act 
symbiotically. The depilating organisms of lime liquors 
are probably mostly anaerobes, but some may be anaerobic 
by adaptation. It is probable that aerobic ferments com- 
mence the depilation, but this will be done before the goods 
are put into work, or at any rate before they reach the 
limes. More strictly, it is the enzymes secreted by bacteria 
which are directly responsible for the hydrolytic work ; 
these enzymes are chiefly proteolytic (proteid splitting), 
but the lipolytic (fat splitting) enzymes have also a place. 

The lime, however, not only limits and selects the 
course of the putrefaction, but also affords more positive 
assistance. Iyime plays its own hydrolytic part and assists 
the depilation by purely chemical action. Lime will unhair 
without the assistance of bacteria, but its action is slow and 
forms a minor part of the operation in the average limeyard. 
This action is due chiefly to its progressive formation of 
calcium sulphydrate from the cystine group of the softer 
keratins. Iyime also plays an essential part in assisting 
the putrefactive fermentation. It softens the keratins 
and thus assists the bacterial attack, it hydrolyzes other 
proteids and provides the bacteria with food in solution, 
the calcium ion increases the proteolytic action of certain 
enzymes, and finally the apparently inert excess of un- 
dissolved lime has an accelerating effect on the bacterial 
activity. 

In the average limeyard these various functions are 
inextricably mixed up, and it is impossible to assign any 
definite proportion of the total depilatory effect to any of 
the factors at work. Iyime alone will unhair, bacteria alone 



22 ANIMAL PROTEINS 

will unhair, and sulphides will also unhair without lime or 
bacteria, but in the limeyard all three agencies are at work. 
Putrefactive fermentation, however, obtains a good start. 
^Evrobic fermentation commences with the slaughter of the 
animal, and the anaerobic organisms soon commence their 
part, and are at work in the hide house and soaks. On 
entering the limes, the purely chemical hydrolytic action 
of lime is added to that of the bacterial enzymes as well 
as the action of lime as bacterial assistant, and the three 
continue to operate side by side. Each gives rise to the 
formation of calcium sulphydrate, whose own special 
solvent effect is superadded. If sulphydrates be deliberately 
added to the liquors there is yet another factor assisting. 
Speaking broadly, the bacterial enzymes have their maxi- 
mum activity in the old limes, and the chemical action of 
sulphydrate formed from the keratin cystine is also at a 
maximum in these liquors. The chemical action of added 
sulphide, and the simple hydrolytic action of calcium 
hydrate have their maximum activity in the new limes. 
Most observers would agree that in practice the bacteria 
shoulder the greater part of the work. 

From the lime3 T ard is taken about the only waste bye- 
products of the tannery, viz. the residues fiom the soak 
and lime pits. These consist mainly of lime and chalk, with 
some hair and dung, and possibly a little sulphide. The 
sludge possesses some value as a manure, especially if from 
the soak pits on account of the gieater nitrogen content. 
(Part VI., Section I.) 

The Beam House Work consists in the mechanical 
removal of those parts of the hide not wanted for leather 
manufacture. Unhairing removes the hair and the epider- 
mis made loose in liming. The hides are placed over a 
sloping " beam " with a convex surface, and the hair 
scraped off with a blunt concave and double-handled knife. 
The hides are then thrown into a pit of water. The hair 
is carefully collected, washed well with water, preferably 
centrifuged, and then dried out by a current of warm air. 
It forms a valuable bye-product. White hair is usually 



THE PREPARATION OF PELT 23 

kept separate and fetches a higher price. Fleshing is the 
next process. The hides are again placed over a beam, 
with the flesh side {i.e. the side nearest the flesh) upper- 
most. Skilled workmen then cut off, with a sharp convex 
knife, the fat, flesh and connective tissue left in flaying. 
Rounding is usually the next process. The unhaired and 
fleshed hide is spread out flat and cut up into butt, shoulder 
and a pair of bellies. These parts have different commercial 
values, and may afterwards be tanned by different methods 
for very different purposes — for dressing leather, and some- 
times even for sole leather. Scudding is the last piece of 
beam work. The fleshed hides (whether rounded or not) are 
washed, or at least rinsed, with water, and again placed on 
the beam grain side up. They are then scraped with a 
rather sharp concave knife, to remove " scud," which consists 
of hair roots and sheaths, lime soaps, fat, pigment and other 
dirt. Short hair is shaved off by a very sharp hand knife. 

The beam work demands a certain amount of skill from 
the workmen, especially from the flesher, whose sharp 
knife may prove very wasteful in incompetent hands. 
Hand labour was slowly but surely being replaced by 
machinery before the war, and war-time conditions have 
greatly accelerated the rate of transition. Beam house 
machinery is rapidly becoming universal. The machines 
are cumbrous and expensive in cost and in power, but 
machine work is quicker, less laborious, and needs much 
fewer workmen. Many types of machine have been sug- 
gested, but the most useful are those in which the hides 
pass over rollers and are simultaneously acted upon by a 
rapidly revolving cylindrical knife with spiral blades, one 
half being a left-handed and the other a right-handed spiral, 
so that the hide is scraped outwards as well as in the direction 
of motion. The part of the hide being acted upon rests on 
a pneumatic roller. By changing the type of spiral knife 
cylinder the machine will unhair, flesh or scud. 

Deliming is a general name covering a number of 
similar operations whose primary object is the neutralization 
and removal of the caustic lime and soda in the plumped 



24 ANIMAL PROTEINS 

pelt, or at any rate on the surface of the hide. This is a 
preparation for the tan liquors. All the tannins and many 
associated substances darken rapidly with oxidation when 
in alkaline solution, so that to place the fully limed hide 
in a tan liquor would give a dark-coloured leather. A 
short insertion in a bath of weak acid would secure the 
elimination of surface lime and the disappearance of this 
difficulty, but there are other purposes in deliming. The 
more completely lime is removed the more the plumped 
pelt " falls " into a soft, pliable, unswollen and relaxed 
condition, and this change assists very materially in the 
production of a soft dressing leather, suitable for boot 
uppers, bags, etc. For such leathers, therefore, the deliming 
must be much more complete than for sole leather, in which 
the object is to obtain a firm and plump leather. 

In the case of the softer dressing leathers, experience 
indicates the advisability of allowing some further bacterial 
action on the interfibrillar substance in order to produce 
the requisite pliability and softness. This is secured by 
" bating " the hides. This process consists in immersing 
the goods into a cold fermenting infusion of hen or pigeon 
dung. The infusion is made in a special tub or pit -with 
warm water and allowed to stand for a day or two until 
the fermentation has commenced, and then run into the 
bating pit through a coarse filter such as sacking. The 
hides are immersed for some days, but are handled fre- 
quently to ensure an even effect. The bate is always 
slightly alkaline. The caustic alkalinity increases rapidly at 
first owing to the diffusion of caustic lime, then at a slower 
rate, afterwards slowly declining. This is explained by the 
production of organic acids, and their salts with weak bases 
from the dung infusion by the action of bacteria. The total 
alkalinity of the bate liquor increases rapidly at first owing to 
the diffusion of lime and its liberation of organic bases, then 
very slowly, but towards the end of the operation the total 
alkalinity increases very rapidly indeed, owing probably to 
the commencement of a violent anaerobic fermentation which 
produces ammonia and other organic bases, and which 



THE PREPARATION OF PELT 25 

heralds the approach of a putrefactive action, which if 
allowed to continue for even a short time will ruin the 
hides. Bating is consequently a risky process, and needs 
experienced oversight. For goods which need only a mild 
bating, there is the alternative of giving a longer liming in 
older limes. This of course involves more bacterial 
hydrolysis, and perhaps does it in a safer, more economical 
and certainly in a less offensive manner. Bating is often 
followed by a further deliming by acids. Boric, lactic, 
acetic, formic and butyric acids are all used, and with care 
even hydrochloric and sulphuric acids may be employed. 
Innumerable " artificial " bates have been put on the market, 
but most are merely weak acids, acid salts or salts of strong 
acids with weak bases. An American " bacterial bate " 
consists of a lactic fermentation of glucose in the presence 
of glue. 

Closely similar to bating is "puering," investigated by 
Wood (see p. 94). 

Drenching is another fermentive deliming process. In 
this the goods are inserted into an infusion of bran. This 
is made by scalding the bran with hot water, and allowing 
it to stand until it is about 70°-c)0 o F. The infusion is then 
" inoculated " with a few gallons of old drench liquor, 
and the goods are immersed. This fermentation has been 
examined carefully by J. T. Wood. First the enzyme 
cerealin converts bran starch into glucose, which is then 
fermented by the drench bacteria with the production of 
lactic acid, some acetic acid and small amounts of formic and 
butyric acids. The butyric fermentation is liable to become 
too violent. These acids, as they are formed, neutralize the 
lime in the hides and plump the pelt slightly (see pp. 
107-109). 

Various gases (carbon dioxide, hydrogen, nitrogen, 
methane and sulphuretted hydrogen) are involved, and the 
proportion produced in the pelt itself has a peculiar opening 
effect on the hide fibres. The activity of the drench can be 
decreased by dilution and by using a less starchy bran, 
and can be increased by adding pea meal or rye meal. 



26 ANIMAL PROTEINS 

Prenching usually follows bating. Scudding sometimes 
follows deliming. 

The theory of the volume and elasticity changes of pelt 
during preparation will be better understood after consider- 
ing the behaviour of gelatine gels (pp. 200-219). The 
determining factors are the nett charge of hydroxyl ions on 
the disperse phase, resulting from ionic adsorptions, and the 
lyotrope influence of dissolved substances on the continuous 
phase. 

In softening dried hides the swelling may be due to 
either influence, but the latter tends to loss of hide substance 
and the production of soft leather. 

In liming, the nett adsorption of hydroxyl ions is the 
principal factor, but the lyotrope influence of the alkali 
cations and of the impurities is important. Plump pelts 
are those in which the contained water is in a relatively 
greater average state of compression. Few substances can 
assist plumping, but many can hinder it. In plumping all 
lyotrope influence is objectionable, and "sharp" (pure) 
alkali solutions are required. Mellow limes reduce elasticity 
and plumpness by lyotrope influence. 

In bating and puering the essential change is that before 
the process the swelling is due chiefly to adsorption of 
hydroxyl ions, whereas afterwards it is due chiefly to a 
composite lyotrope influence. 

REFERENCES. 

" Principles of Leather Manufacture," Procter, pp. 108-184. 

" The Manufacture of Leather," Bennett, pp. 49-113. 

" Lyotrope Influence and Adsorption in the Theory of Wetwork," 
Bennett, J.S.L.T.C., 1920, pp. 75-86. 

"Analytical Examination of Bating," Bennett, Leather Trades 
Review, 191 1, p. 972, and 1912, p. 28. 

"The Bating, Puering and Drenching of Skins," by J. T. Wood. 



Section III.— VEGETABLE TANNAGE 

Au, tannages have for their object the conversion of the 
readily putrescible hide tissue of the corium (the pelt) into 
an imputrescible, insoluble and permanent material called 
" leather," which, possessing considerable strength and 
pliability, is capable of application to a variety of useful 
purposes. The conditions necessary for this transformation 
have been clearly stated by Procter. 1 For the production 
of leather from pelt "it is not only necessary to dry the 
fibres in a separate and non-adherent condition, but so to 
coat them or alter their chemical character that they are 
no longer capable of being swelled or rendered sticky by 
water." Whatever substance will secure this permanent 
dehydration of the hide fibres in a separate condition is 
called a " tanning material." The change from pelt to 
leather is known as " tannage," the process is termed 
" tanning," and those who undertake it are " tanners." 

In " vegetable tannage " the tanning materials are of 
vegetable origin, and contain a group of organic compounds 
called " tannins " which are extracted by the infusion of 
these materials with water. Pelt, when immersed in these 
infusions, is converted into leather, rather slowly ; but a 
gelatin solution gives an immediate precipitate of " amor- 
phous leather," even if the tannin infusion be exceedingly 
dilute. The tannins are aromatic compounds of phenolic 
character, and contain carbon, hydrogen and oxygen only, 
but our knowledge of their chemical constitution is exceed- 
ingly small owing to their instability and colloid nature, 
which make impossible their preparation in a pure state. 
They are all, however, derived from either catechol or 
pyrogallol, and yield these substances if carefully heated to 

1 " Principles of Leather Manufacture," p. 184. 



28 ANIMAL PROTEINS 

about 200° C. The tannins are soluble in water, alcohol, 
acetone, ethyl acetate and acetic acid, but insoluble in 
benzene, chloroform, carbon disulphide, petroleum ether, 
dilute sulphuric acid and pure ethyl ether. The aqueous 
infusions of the tannins are in reality colloidal solutions ; i.e. 
heterogeneous systems of two phases. The systems are 
lyophile, or, more particularly, hydrophile, i.e. there is an 
affinity between the two phases. As usual with lyophile 
systems the two phases may be considered as both liquid, 
and an aqueous infusion of tannin forms an emulsoid sol, 
which therefore is subject to the phenomenon of adsorption. 
The tannins are all precipitated by solutions of basic lead 
acetate and copper acetate, and many of them with varying 
completeness by solutions of many other metallic salts and 
hydroxides, of basic dyestuffs and of alkaloids. They give 
dark colorations with ferric salts. 

The tannins are widely distributed in plant-life, but 
only in a limited number of cases do the plants contain 
sufficient tannin to render them of commercial importance. 
Tannin is found in all parts of plants, but usually in greatest 
amount in the bark or fruit. The tannins are classified 
into " pyrogallol tans " and " catechol tans," according to 
the parent phenol. This classification is confirmed by their 
chemical, analytical and practical behaviour, and the 
vegetable tanning materials may be classified into the 
same two groups, for, although even the same plant contains 
both pyrogallol and catechol tans, it is usual to find in any 
one part of the plant that one group is predominant. 

Pyrogallol tans, which are oftenest obtained from 
fruit or leaves, contain usually about 52 per cent, of carbon. 
Used alone they produce a rather soft and porous leather. 
Associated with them — in many cases probably as decom- 
position products — are certain other substances of well- 
known properties and constitution. These substances are 
not only typical of the group, but also form the most valuable 
clue to the chemical constitution of the group and the key 
to their chemical behaviour. One of these substances is 
gallic acid (3:4:5 trihydroxy-benzoic acid C 6 H 2 (OH) 3 - 



VEGETABLE TANNAGE 29 

COOH, which possesses properties very similar to the 
tannins, but does not precipitate gelatin and will not itself 
make leather. Another of these substances is ellagic acid, 
Ci 4 H 6 8 , a double lactone of a hexa-hydroxy-diphenyldi- 
carboxylic acid. This is deposited as an insoluble yellow 
powder from infusions of many pyrogallol tans, by boiling 
with dilute acids only, allowing them to stand for a few 
days. In practice the deposit is found as mud at the bottom 
of the tan pits, and also upon the leather, to which it strongly 
adheres. It is technically known as " bloom." It is 
insoluble in acids and cold alcohol, but soluble in alkalies. 
It is a feeble dye-stuff. The pyrogallol tans yield very 
different amounts of bloom. Other associated substances 
are the sugars. In practice these sugars ferment to lactic, 
acetic, and other acids which cause " sour '-' liquors. Such 
liquors plump the hides and tend to give firm, thick leather. 
These acids also probably cause increase of adsorption 
of tannin by the hide and therefore assist in giving " good 
weight." Solutions of pyrogallol tans all give a blue-black 
colour with a dilute solution of ferric alum. If a solution 
of sodium arsenate be added to an infusion of pyrogallol 
tan diluted until no longer distinctly coloured, and the 
mixture allowed to stand for about two hours, a green 
colour develops at the surface of the liquid. The reaction 
is due to gallic acid or a similar grouping, and is, in the 
author's experience, the most satisfactory qualitative test 
for the group. Another test is to mix equal volumes of a 
0*4 per cent, infusion of tan and a 10 per cent, solution of 
sodium bisulphite ; a few drops of 10 per cent, potassium 
chromate are added, and either a transient blood-red colour 
or a more permanent deep purple is obtained. The former 
colour is due to gallic acid. If a tannin infusion be largely 
diluted with hard water and a little iodine solution added, 
the pyrogallol tans yield either a purple-red or a dark 
blue colour, the former being a reaction of gallic acid. 
Pyrogallol tans yield no precipitate with bromine water. 
They yield a yellow or brown colour when one drop of 
infusion is added to concentrated sulphuric acid. 



30 ANIMAL PROTEINS 

Myrabolans is one of the most important of the 
pyrogallol tanning materials. It is a name given to the 
dried fruit of Terminalia chebula and other species of Indian 
trees. The nuts resemble an elongated walnut. They are 
dried and exported from many parts of India to all parts 
of the world, but largely to this country. The varieties of 
commerce are named according to origin and quality : 
thus we have " Ji's," i.e. Jubbelpore, No. i quality, " Ri's " 
(Rajpore, No. i), " Bi's " (Bhimley, No. i), etc. The 
little difference in tannin strength (about 32 per cent.) in 
these varieties is usually compensated by corresponding 
differences in price. The quality of myrabolans cannot be 
safely judged by appearance. Much bloom is deposited by 
myrabolans liquors, especially by " J's." Myrabolans are 
amongst the most sugary of tanning materials, containing 
up to 5 § per cent. It is therefore one of the best materials 
for giving a plump leather. Broadly speaking, those varieties 
which yield most sugar yield least bloom, and vice 
versa. Myrabolans tannin has a small affinity for hide 
substance and penetrates the hide very slowly. It gives 
a " mellow " tannage, but a bright, good colour, which 
characteristics are imparted to the leather when the material 
is blended with other materials containing dark or astringent 
tannins. When used alone it yields a rather spongy 
leather, and it is not considered a good weight-giving 
material, though its acid-producing powers are very helpful 
to other more astringent tannins. 

Valonia has been the other staple tanning material of 
the heavy leather trade. It is the acorn cup of oaks 
common in Asia Minor and Greece, chiefly the Turkish 
oak (Quercus cegilops). The fruit is gathered when ripe 
and dried in layers of about one foot deep until the acorn 
drops out, Smyrna is the great export centre. Greek 
valonia is obtained from many parts of the Archipelago 
and mainland. It is gathered in a more immature con- 
dition and includes tne acorn. It is considered slightly 
inferior in strength and colour to the Smyrna valonia. 
The exterior of the acorn cup is covered with rather scaly 



VEGETABLE TANNAGE 31 

protuberances known as " beard," which contains usually 
about 40 per cent, of tannin. The cup alone contains 
usually about 25 per cent, tannin, and the whole about 
30 per cent. The valonia tannin has been thought to 
contain two chemical individuals, only one of which produces 
bloom. Parker and I^each x found that the tannin of the 
cup produces more bloom than that of the beard, and that 
Smyrna valonia yields more bloom than Greek. The more 
bloom is deposited, the less acid will be produced. Under 
all conditions the yield of bloom is large, and its deposition 
in and on the leather assists materially in giving the weight 
and water-resisting powers associated with sole leather 
which has been largely tanned with valonia. The valonia 
tannins have only a moderate affinity for hide, which, like 
myrabolans, they penetrate very slowly. When used alone 
the leather is less yellow than that from myrabolans, and 
is also duller. After most of its bloom has been deposited 
valonia makes a very suitable tannage for dressing leather, 
and in conjunction with gambier has been largely thus 
used. Since the outbreak of war the Turkish product has, 
of course, not been available for importation. 

Sumach 2 is the other pyrogallol tan of commercial 
importance. It consists of the leaves and small twigs of 
the Sicilian sumach (Rhus coriaria) cultivated in Italy 
extensively for export. The leaves are hand picked, dried 
and often ground to powder. It contains 26-28 per cent, of 
a tannin which yields little or no bloom, but much gallic acid. 
It is an unstable tannin, and its infusion rapidly ferments. 
Sumach is a very valuable tanning material, and when used 
alone gives an exceedingly durable leather of excellent light 
colour. It gives a soft mellow tannage, and is therefore 
most suitable for light leather tanning, and is extensively 
used for this purpose. It is used, nevertheless, in large 
quantities by the heavy leather tanners for finishing pur- 
poses, for it contains some organic reducing agent which 

1 J.S.S.I., 1903, 1 184. 

2 Also spelt Sumac and Shumac, and always pronounced like the 
latter. 



32 ANIMAL PROTEINS 

exerts a powerful bleaching action on other tannages, and 
which assists to brighten as well as lighten the rather dull 
appearance of leathers largely tanned with valonia. It is 
rather an expensive tannin, but most manufacturers find 
that its results are worth its cost. 

Other pyrogallol tans are also used to a limited extent. 
Algarobilla and divi-divi are the fruit pods of several species 
of American ccesalpina. They are strong in tan (45 per 
cent.) and yield a light-coloured and bright leather, but are 
unstable tans, yielding much bloom. Babla is a small 
pod yielding a mellow tannage and much gallic acid. Cela- 
vinia is another pod containing no colouring matter and 
giving an almost wtiite leather. The tannin is closely 
similar to that of oak galls. These last were once exten- 
sively used for tanning in Austria. Willow bark is used for 
tanning in Russia and Denmark. Valuable pyrogallol 
tannins are obtained from oak wood and chestnut wood, 
but the woods are not used in tanning as the percentage of 
tan is so small. 

Catechol tans, often obtained from barks, contain 
usually about 60 per cent, of carbon. They are seldom 
used alone, for they usually have little or no sugar associated, 
and hence their liquors do not either " sour " or " plump." 
They can be used alone if artificially acidified, but without 
acidifying or blending would give a rather flat leather, 
though possibly firm. They )deld no bloom or gallic acid, 
but have associated with this other characteristic substances. 
Of these the catechins are the most typical, and have been 
considered as the parent substances of the catechol tans. 

The catechins are white crystalline substances, ap- 
parently isomers with the general formula C 18 H 14 6 . They 
have different melting-points, and varying amounts of water 
of crystallization, but are otherwise exceedingly similar in 
properties. They are sparingly soluble in cold water, but 
freely in hot, and in alcohol and ether. They are pre- 
cipitated by lead acetate, mercuric chloride and albumin, 
but not by gelatin, tartar emetic or alkaloids. In gambier 
liquors they are especially strong, and sometimes crystallize 



VEGETABLE TANNAGE 33 

on the side of the pits, being thus known as " whites." 
The phlobaphenes or " reds " are also typical of catechol 
tans from which grow catechins ; they can be formed by 
boiling with dilute mineral acids. They are considered 
to be anhydrides of the catechol tans. They are difficultly 
soluble in cold water, but freely in hot, and in cold alcohol 
and dilute alkalies. They are true tannins and alone are 
capable of making a red leather, but in practice are often 
found as mud in the tan liquors owing to their limited 
solubility. They naturally influence the colour of leather 
made with catechol tans, which is usually distinctly redder 
than the leather made from pyrogallol tans. Infusions of 
catechol (cp. catechin) give a green-black colour with iron 
alum. The sodium arsenate test gives a red colour due to 
catechin. The chromate and iodine tests mentioned for 
pyrogallol tans give negative results with the catechol tans, 
but bromine water gives a precipitate, and sulphuric acid 
a crimson colour. 

Mimosa bark is one of the most important catechol 
tans. It is usually obtained in this country from Natal 
(" Natal bark ") ; but the tree (Sydney green wattle, Acacia 
mollissima) is a native of Australia. It is being cultivated 
now extensively in South Africa, and forms a most valuable 
portion of the Empire's stock of tanning material. Its 
more extensive use has been long recommended by the 
author, 1 but its gradually increasing employment in British 
tanneries has been greatly accelerated by the war, which 
has prevented its delivery in Germany and has cut off 
Turkish valonia from Britain. It yields about 30 per cent, 
of a stable and excellent tannin, and will produce a firm, 
durable leather, with a colour much less red than that 
obtained from many other catechol tans. It is an astringent 
tan, and if carelessly used yields a harsh or even " drawn " 
grain. Most of the tannin is easily extracted, yielding a 
clear infusion which penetrates fairly quickly and gives 
good weight. It contains less than 1 per cent, of sugar, 
which unfortunately rapidly ferments to carbonic acid, so 
1 J.S.C.I., 1908, 1193. 
K. 3 



34 ANIMAL PROTEINS 

that it is not a good plumping material. It makes in all 
respects an excellent blend with myrabolans. Iyike all 
catechol tans, the resulting leather darkens on exposure to 
sunlight. 

Oak bark, from Quercus robur, is the ancient tanning 
material of Britain, and is still used to a limited extent. 
It contains about 13 per cent, of tannin and is mainly a 
catechol tan, but also contains a pyrogallol derivative. 
It yields catechin, and gives a red colour with the sodium 
arsenate test, but also will yield some bloom and gallic acid, 
and gives a blue-black with ferric salts. The tannin itself 
is exceedingly similar to that of mimosa bark, but the 
material contains about 2§ per cent, of sugar, which makes 
it possible to employ oak bark alone for making sole leather. 
It is noted for yielding a sound, durable leather of good 
typical tan colour. Its tannin combines well with hide 
and penetrates quickly. The fatal disadvantage of oak 
bark is its weakness in tannin strength compared with 
other materials. This results in heavy freight and heavy 
cost per unit tannin, bulky storage, expensive handling in 
the factory, comparatively large bulk of spent tan, after 
relatively greater trouble in extracting, and the impossi- 
bility of making the strong liquors so necessary in these 
days to produce good weight in a short time. No satis- 
factory extract has yet been made from it. 

Pine bark, from Pinas abies, is one of the staple materials 
of the Continent. It contains up to 14 per cent, of a catechol 
tan, and, unlike most of this group, contains a high propor- 
tion of sugar and will give good results alone. Hemlock 
bark has been the staple tanning material of North America. 
It is obtained from the hemlock, or Pinus canadensis. It 
contains up to 11 per cent, of tan and much phlobaphene, 
and yields a characteristic red leather of good quality, but 
which rapidly darkens with sunlight. It contains some 
sugar, but is usually employed in conjunction with sulphuric 
acid or with sugary materials. Mallet bark yields another 
catechol tan similar to that of mimosa, but somewhat less 
astringent and more yellow in colour. Quebracho wood 



VEGETABLE TANNAGE 35 

and mangrove bark have been used, but are now made into 
extracts (pp. 38 and 41). 

Leaching. — Whatever class of leather is being made, 
and whatever blend of tanning materials is being employed, 
the tannins must be efficiently extracted by water in order 
to make the tanning liquors. This process is called " leach- 
ing." The tanning materials, after being ground, crushed 
or shredded, are placed in large pits arranged in " rounds," 
" sets," or " batteries " of 6, 8 or 10 units, through which 
water is percolated systematically, so as to secure a con- 
tinuous extraction. Water itself is added to only one of 
the pits of material. The liquor produced is passed on to 
the next pit, and then to the next, and is continually gather- 
ing strength. After passing thus through the series, the 
liquor becomes the source of the strong extracted tan 
liquors which are used in the tannery proper. With this 
system the stronger leach liquors are being acted upon by 
fresh material, and the nearly " spent " material is being 
acted on by the weakest liquors, and finally by water, thus 
ensuring a complete extraction. In the press leach system, 
which is now practically universal, the bottom of one pit 
communicates with the top of the next, and the liquor 
presses round by gravity flow caused by a few inches " fall." 
1,-iquor is thus constantly percolating downward through 
the material in each pit. The " head leach " and " tail 
leach " are always adjacent in a double row of pits, and 
when the material in the latter is quite spent, it is " cast," 
and the pit is filled with fresh material. The liquor is then 
pressed round into this pit by adding water to the tail 
leach. Hot water is used to secure better diffusion. At 
least two such sets of leaches (" taps " and " spenders ") 
are necessary to spend the material of the average tannery 
and to obtain liquors of the necessary strength. 

The Manufacture of Extracts.— In addition to the 
use of the natural tanning materials described above, 
modern leather manufacturers employ also a variety of 
" tanning extracts," i.e. vegetable tanning materials in 
which the tannin has been already extracted, and which 



36 ANIMAL PROTEINS 

are supplied in form of a solid or concentrated liquid. Such 
extracts only need to be dissolved in warm water in order 
to make a tan liquor, and the cost and trouble of leaching 
is avoided. They are a great convenience as making 
strong liquors of definite strength. Many vegetable tanning 
materials are too weak in tan for the tanner to leach, and 
indeed to justify the cost of importation have been made 
available by manufacturing an extract at the source of the 
material. With such weak materials the extract manu- 
facturer has had to secure a much more complete extraction 
than in ordinary leaching, and to concentrate his infusions 
by means of steam-heated vacuum pans. With such 
experience he has naturally begun to make extracts also 
from the stronger materials, such as myrabolans and 
mimosa bark, and it is now possible to have a tannery 
without any leaches at all. Tanners also have begun to 
realize the advantages not only of more rapid and complete 
extraction, but also of doing the work for themselves, and 
extract factories are beginning to appear as an adjunct to the 
larger tanneries. The more complete extraction of tan also 
involves a greater extraction of unwanted colouring matters, 
hence decolorization is a feature of extract manufacture. 

Chestnut Extract is from the wood of the Spanish 
chestnut {Castanca vesca), which contains 3-6 per cent, of 
a valuable pyrogallol tan very similar to that of valonia. 
Its weight-giving and water-resisting powers are as good 
as valonia, and its penetrating power is even better, so that 
it forms an exceedingly suitable material for the modern 
short tannage, and also for drum tannages. The extract 
is manufactured extensively in France. The wood is stripped 
of bark and usually piled for some months to dry and to 
allow the resins to become insoluble. Some factories, 
however, use the green wood direct. There are two methods 
of extraction, viz. in open vats and in closed vats under 
pressure. The two methods yield extracts which differ in 
composition and properties. In either case the vats have 
a capacity of up to 3000 gallons, and hold up to 6| tons 
of wood. They are arranged in series, as in leaching, and 



VEGETABLE TANNAGE 37 

the liquor passes in succession through all the vats over 
wood less and less spent. The temperature is highest in the 
vat containing the fresh water and nearly spent wood. In 
open vats of wood or copper the temperature is near boiling- 
point, whilst in the closed autoclaves (copper or bronze) 
the pressure reaches about two atmospheres and the tem- 
perature about 130 C. (266 F.). The series may contain 
5, 7, g or even 12 vats, and the liquor obtained has a strength 
of 3 to 4|° Beaume (22 ro 33 Bkr.). 

After extraction the liquor is allowed to stand, and 
much insoluble matter settles out — resins, wood, fibre, etc. 
The clarified and settled liquor is then passed through a 
cooler up to about 55 C, and then run into the decolorizing 
plant, a deep vat fitted with a copper steam coil and 
mechanical stirrer attached to power. The best decolorizer 
is bullock's blood, which is run into the vat and well mixed. 
The temperature is next raised to about 70 C, causing the 
blood albumin to coagulate. It carries down with it a 
little tannin, but much colouring matter. After standing 
a few hours the settled liquor is run oft" direct to the 
evaporator. A multiple-effect evaporator is usually em- 
ployed, and the concentrated liquor, which has a strength 
of about 25 Beaurne, is run into suitable oak casks. The 
extracts contain 27-32 per cent, tannin. An extract made 
with open vats has about 7 per cent, soluble non-tanning 
matters, whilst a " pressure extract " may contain up to 
12 per cent, of these " non-tans." Pressure extracts obtain 
also a better yield of tannin, which more than compensates 
for the slightly lower price. Open extraction }delds, how- 
ever, the purer product and an extract with better pene- 
trating powers, and is consequently the more suitable for 
drum tannages. Chestnut extract is extensively used by 
the heavy leather tanners. 

Oakwood Extract is manufactured from the wood of 
the common oak (Quercus robur) . The centre of the industry 
has been the oak forest of Slavonia. The wood contains 
2-4 per cent, of a tannin very similar to that of chestnut 
wood, but somewhat more astringent. 



38 ANIMAL PROTEINS 

The manufacture is also similar to that of chestnut 
extract, but decolorization is often omitted, and greater 
care has to be taken and in other ways to keep the colour 
within limits. One of these is to strip the wood more 
completely of bark. Another is to operate at as low a 
temperature as possible, about no C. The extraction is 
made in large circular vats about 14 feet high and holding 
about two tons material. A battery is composed of about 
eight vats or extractors. Open extraction is used, and 
the liquor is passed forward after 2-3 hours' boiling, so 
that the material is spent in about 24 hours. A liquor of 
about 5 Be (36 Bkr.) is obtained, and the strength of the 
material reduced from 4 to J per cent, of tannin. Getting 
rid of insoluble matter is a difficulty, and is attained by 
settling, by rapidly cooling, and then passing through a 
filter press of wood. For evaporation a double -effect 
vacuum pan is preferred, which operates first at about 
113 F., and afterwards at 140 F. with a higher vacuum. 
The liquor is concentrated from 5 to 25 Beaume (s.g. 
1-036 and 1 "210 respectively). 

The extract has a much higher colour than chestnut, 
and is not used now as much as some years ago. As the 
principal supply was German, it has been unavailable. 

Quebracho Extract is made from the wood of the 
South American tree Loxopteryngium Lorenzii, which con- 
tains about 20 per cent, of a typical catechol tan. It is 
associated with a little catechin, much phlobaphene, but 
practically no sugar. The tannin is very astringent, 
penetrates quickly and gives a firm red leather which darkens 
on exposure to light. It is not noted for weight-giving 
powers. The wood itself, as chips or shavings, has been 
used in British tanneries, to a limited extent, but the great 
bulk of the material is made into extract chiefly in South 
America. The crude " extract," made by evaporating 
aqueous infusions of the wood, is largely exported for 
refinement in Europe. It is also refined on the spot to a 
large extent and converted into solid extract containing 
60 per cent, of tannin. 

The great difficulty with quebracho has been the disposal 



VEGETABLE TANNAGE 39 

of the phlobaphenes, and a great variety of quebracho 
extracts are now available which deal with this problem 
in different ways. In some the more soluble reds are simply 
left in the extract under the idea that they are really tannins 
and may be of some use in some part of the tanning process ; 
in others they have been removed by settling and filtration at 
appropriate temperatures and concentrations ; in most, how- 
ever, they have been solubilized by treatment with alkalies, in 
the presence of reducing agents, notably by heating with 
sodium bisulphite in closed vats. The base combines with 
the phlobaphenes, which are made completely soluble and 
available for tanning. Sulphurous acid is evolved, and its 
reducing powers assist materially in retaining and promoting 
a good colour in the product. Such " sulphited extracts " 
are now extensively manufactured in this country from the 
imported " crude " extract, and sold as liquid extracts con- 
taining 30, 35 or 40 per cent, of tan according to the require- 
ments of the buyer ; " mixed extracts," which are solubilized 
quebracho blended with about 15 per cent, of myrabolans, 
are also used. 

By solubilizing quebracho with excess of bisulphite an 
extract is obtained which possesses considerable bleaching 
powers, and such extracts are also extensively manufactured 
for the " vatting " or bleaching of heavy leather after tannage. 
The excess of sulphurous acid not only bleaches the leather, 
but also swells it up and thus permits a further absorption of 
strong tan liquor, which is conducive to good weight. These 
bleachingextracts areusually of 36-38 percent, strength in tan. 

Gambier is an extract of the leaves and twigs of the 
eastern shrub Nauclea gambir. It is a catechol tan of 
peculiarly mellow quality and great practical value. It 
contains much catechin, but little phlobaphene, and yields 
a beautifully soft leather, but without weight. It is an 
exceedingly suitable material for the early stages of tanning, 
and is much liked for tanning leathers that have to 
be curried, and is widely used in the manufacture of 
upper leather. It is, however, an exceedingly expensive 
tannin, and the extract is made in a very crude way by 



40 ANIMAL PROTEINS 

Chinese and Malays without much supervision. Hence its 
strength in tan and general quality is extremely variable. 
The plant is cultivated for the purpose of extract manu- 
facture, and prunings are taken in the plant's third year. 
They are bruised and boiled with water in the open. The 
infusion is strained, concentrated, and poured into cooling 
vessels in which it sets to a paste. Two varieties of gambier 
are well known, " cube gambier " and " block gambier." 
In the latter the extract remains as a paste containing 25 
to 40 per cent, of tannin. It is sold in oblong blocks of 
1 or 2 cwt., either wrapped in cocoanut matting or in 
wooden boxes. Cube gambier is made by running the con- 
centrated syrup into trays 2 inches deep and drying in the 
sun. When partly dry, it is cut up into ij-in. cubes and 
dried further on cocoanut matting. The rough " cubes " 
as imported contain 40-50 per cent, of tannin. 

Myrabolans Extract is now largely manufactured in 
this country. A liquid extract of 25, 30 or even 35 per 
cent, strength is made for home consumption, and a solid 
extract for export. The light colour, high strength and 
easy extraction of the natural material have all facilitated 
the task of the manufacturer. 

The material is extracted in open vats or stills of copper, 
which take one ton or more of nuts. A battery of 4, 6 or 8 
of such stills is usually employed, and the temperature is 
kept well below boiling-point except in the vats containing 
the nearly spent material. The liquors move 'forward 
quickly, and the material is quite spent in 24 hours. The 
material when cast contains less than J per cent, of tannin. 
The liquor obtained is 40°-50° Bkr. (6— 7J per cent, tan), 
and after settling is concentrated at 40°-50° F. in a single 
effect vacuum pan, which though more costly in steam is 
quicker than the multiple effects, and gives the low tem- 
perature required. For solid extract the more concentrated 
liquor is run direct into tarred bags, in which it soon solidifies. 

Hemlock Extract is manufactured from the North 
American pines and imported into this country to some 
extent. It gives a very red colour. 



VEGETABLE TANNAGE 41 

Mangrove Extract is made from the bark of Rhizophona 
Mangle and other species of mangrove which grow freely 
in the tropical swamps of West Africa, Borneo, etc. Much 
solid and liquid extract has been made from this material, 
but is not very popular on account of its harsh tannage and 
dark red colour. 

Pine Bark Extract (Larch extract) is made in Sweden 
from the Norway spruce (Pinus abies). It is slightly 
sulphited and gives a good colour. It is a liquid extract 
of about 30 per cent, strength, and is sometimes used as a 
chestnut substitute. It should not be confused with the 
so-called " spruce " or " pine wood " extract, which is a 
paper trade bye-product and contains ligneous matters 
rather than tannin. 

American Chestnut Extract, made from the chestnut 
oak, is either a liquid or a solid extract in powder form. It 
gives a wretched brown-black colour, which is quite unsuited 
to the usual British needs. 

THEORY OF VEGETABLE TANNAGE. 

Vegetable tannage is a phenomenon of colloid chemistry. 
The old arguments as to whether tanning was a chemical or 
a physical process have been rendered obsolete by the advent 
of a new set of explanations, which, though shedding light on 
many obscure points, have enormously increased the com- 
plexity of the problem. In vegetable tannage an emulsoid 
gel (pelt) is immersed in a complex emulsoid sol (tan liquor), 
which immersion results, not in simple reaction or change, 
but in a series of changes. 

One of these changes is adsorption. Pelt is a gel which 
possesses a great development of surface. It not only 
exhibits like gelatine the phenomenon of imbibition and 
dehydration to a very marked extent, but also possesses 
a very fine fibrous structure due to its organic origin ; thus 
pelt possesses an enormous specific surface, further intensified 
by the preparation processes previously discussed, which 
split up the hide fibres into smaller bundles and into much 



42 ANIMAL PROTEINS 

finer constituent fibrils. Tannins, on the other hand, are 
hydrophile colloids which in water form emulsoid sols, and 
which may thus be expected to exhibit the phenomenon of 
adsorption. A tan liquor usually contains several tannins 
in addition to other closely similar substances, also in 
colloidal solution, and is therefore a sol of considerable com- 
plexity. The immersion of pelt into a tan liquor results in 
an adsorption, which consists essentially in an inequality 
of concentration in the sol, the greater concentration being 
at the interface. This inequality between the surface con- 
centration and the volume concentration of the sol, is due 
primarily to considerations of surface tension and surface 
energy, and exists before the immersion of the pelt. The 
surface layer having excess over the volume concentration, 
any considerable extension of surface in a fixed volume of sol 
must produce a very considerable decrease in the volume 
concentration. This is what occurs when pelt is immersed 
in a tan liquor, the immersion being the considerable exten- 
sion of surface. It should be especially remembered that the 
inequality of concentration is in the sol, on the liquid side of 
the interface. In adsorption, the substance adsorbed, i.e. the 
excess at the surface, is too frequently regarded as bound to 
the solid immersed. This is because the excess is in the layer 
which wets the solid and remains wetting it when the solid 
is removed. Thus the immersion of pelt produces primarily 
only a change in the distribution of the tannins in the liquor. 
It follows from this that the adsorption is an equilibrium, 
and that if the sol be diluted, the equilibrium will become 
the same as it would have been by immersing the pelt 
directly into the dilute solution. Thus, if pelt be first 
immersed in one tan liquor and then into a weaker one it 
will yield tan to the latter solution. 

The chief object in heavy leather tanning is to obtain 
the maximum possible adsorption in the minimum possible 
time, or in other words, to obtain good weight quickly. 
The amount adsorbed is proportional to the actual ex- 
tension of surface, i.e. the adsorption is a function of 
the specific surface of the adsorbent. Hence, to obtain 



VEGETABLE TANNAGE 43 

good weight it is necessary to develop in the pelt its 
maximum possible specific surface. This is one of the 
objects of " plumping," which splits up the fibres. It is 
attained also by the solution of interfibrillar substance in 
limes and bates. 

The amount adsorbed is also a function of the volume 
concentration in the sol after equilibrium is reached. Hence 
the better weights are obtained with stronger liquors. 

The adsorption law is 

y l 

- =ac n 
m 

where y is weight adsorbed by the weight m of adsorbent, 
and c the volume concentration after adsorption ; a and n 
are numeral constants. Hence weight is determined by 
the strength of the liquor which the goods finally leave. 
The commencement of tannage is necessarily in weak 
infusions, in order to secure the maximum diffusion into 
interior of the fibres before they become heavily coated 
on the exterior. As the equilibrium is being established 
in such liquors the volume concentration diminishes, and 
thus makes it less likely that good weight will be attained ; 
hence it is necessary in practice to move the goods con- 
stantly into fresh liquors of gradually increasing strength, 
and so maintain the rate of adsorption and save time. A 

further consequence of the adsorption isotherm is that as 

1 
y varies as c n and n is >i, y is increased appreciably only 
by a relatively large increase in c. Hence, though stronger 
liquors give better weight, there is a limit beyond which 
any further gain in weight is not justified by the enormous 
increase in the concentration necessary to attain it. Such 
great increase in c is impracticable not only on the ground 
of expense, but also on account of the great viscosity of the 
sol. 

The amount of adsorption depends also upon the exact 
nature of the sol. It has been previously pointed out that 
the tannins differ largely in their penetrating and weight- 
giving powers. Some are readily adsorbable and are 



44 ANIMAL PROTEINS 

deposited in great concentration at the surface of the fibre, 
but for good weight it is necessary to use also the less 
adsorbable and more diffusible tans, which penetrate the 
fibre itself. Hence it is necessary for good weight to use 
a blend of materials, and so supply many grades of 
liability to adsorption. It is particularly advantageous 
to blend judiciously the two main types of material, the 
pyrogallol and catechol tans. It is also necessary for good 
weight to present to the pelt the more diffusible and less 
adsorbable tannins first, in order to secure the maximum 
diffusion into the interior of the fibre before the exterior 
of the fiore is heavily coated with the heavily adsorbable 
and astringent tans. The least adsorbable materials are 
therefore used in the early stages of tanning, and the most 
adsorbable materials at the end of the tanning process. 
Thus gambier is added to the early liquors (suspenders), 
solubilized quebracho to the later liquors (handlers), and 
mimosa bark extract to the final liquors (layers). There 
is also another excellent way of ensuring this progressive 
astringency of the liquors; this consists in leaching the 
required blend of materials together (or mixing them in 
the case of extracts) and presenting the mixed infusion to 
the nearly tanned goods, which adsorb chiefly the more 
astringent tannins. The liquor is then used for goods at a 
less advanced stage of tanning, which again take the most 
adsorbable constituents. This is repeated until the stage 
is reached when the fresh pelt is inserted into the nearly 
exhausted liquor, which naturally contains only the least 
adsorbable substances. This system is almost universal, 
and in practice is known as " working the liquors down the 
yard." It has the additional advantage of being a syste- 
matic method of economically exhausting (" spending ") the 
tan liquors. When free acid is present in the tan liquors, 
it tends to distend the fibres composing the pelt by a strong 
and rapid adsorption. Thus distended or plumped the 
fibres present a still greater surface for adsorptive operation, 
but the distension naturally leaves less space between the 
fibres for the diffusion of the sol. Hence acid or " sour " 



VEGETABLE TANNAGE 45 

tan liquors give in the long run more weight, but tan more 
slowly. Pelt tanned whilst thus plumped forms naturally 
a thicker and less pliable leather. This occurs in tanning 
sole leather, to a less extent with heavy dressing leather, 
and to a very small extent in the case of softer dressing 
leathers. 

In addition to adsorption, there is another phenomenon 
of colloid chemistry in operation, viz. the mutual precipita- 
tion of the sols in the liquid by the gels in the hide. In 
most sols the disperse phase is electrically charged. The 
sol therefore possesses electric conductivity, and migration 
occurs in the electric field to the cathode or anode according 
to the nature of the charge. Oppositely charged sols pre- 
cipitate one another, the precipitate containing both colloids. 
The maximum precipitation occurs when the + charge of one 
sol exactly equals and neutralizes the — charge of the 
other. There is thus an electrical equivalence ; an amount 
of sol which is equivalent to a given amount of the other. 
This is not a chemical equivalence, however, and the 
precipitate is not a chemical compound in spite of its fairly 
constant composition. The composition of the precipitate, 
indeed, is not quite constant, for the optimum precipitation 
may not correspond exactly with the electrical equivalence, 
being influenced by the number of particles required, their 
size (dispersity) , the rate of mixture, and the relative con- 
centrations of the sols. This mutual precipitation is ex- 
hibited by emulsoids as well as suspensoids, but the charge 
(+ or — ) on an emulsoid is in many instances largely an 
accidental matter, being determined by the medium in 
which it happens to be, its normal condition being electrical 
neutrality. Gelatin and pelt are such emulsoids, and 
a positively charged gelatin sol has been observed to pre- 
cipitate a negatively charged gelatin sol. It is thought, how- 
ever, that gelatin is primarily a positive sol. Pelt (whether 
delimed or not) is rapidly acidified by the quickly penetrating 
and strongly adsorbed organic acids of the old tan liquors 
and becomes positively charged before the tannins are 
adsorbed. The positive charge increases ■with the acidity 



46 ANIMAL PROTEINS 

of the liquor. Other emulsoids are not electrically neutral, 
but are electrically charged and exhibit considerable con- 
ductivity. Into this class fall the tannins, and in tanning 
it is thought that there is a mutual precipitation of the nega- 
tive tannin sol with the positive hide gel, the precipitation 
of the negative sol being favoured by the acid condition of 
the liquor. The effect of increasing acidity soon falls off, 
however, as a saturation limit is soon reached. This 
mutual precipitation of colloids in tanning is in reality but 
an extension of the adsorption theory, which explains the 
predominant effect of H+, and OH~ on the electric charge 
by stating that these ions are more readily adsorbed than 
other ions, and that as OH - is more readily adsorbed 
than H+ most sols are negative to water. 

In addition to the adsorption phenomena described, 
there are in vegetable tannage secondary changes which 
are slow and "irreversible." These changes are obscure and 
are difficult to investigate. Oxidation, dehydration and 
polymerization have all been suggested, but there is little 
direct evidence. Certain it is, however, that time renders 
the tannage more permanent. It perhaps should be pointed 
out that in the very strongest tan liquors the viscosity of 
the tannin sol is so great that adhesion would be a better 
term than adsorption. There is no abrupt division between 
the two phenomena. 

In the theory of vegetable tannage there is another factor 
the importance of which has been strongly emphasized by 
the author, viz., lyotrope influence. This has been most 
conveniently discussed in connection with gelatin gels (pp. 
200-219), but its effect on hide gels is analogous. It has 
also an effect upon the diffusion and gelation of the tannin 
and non-tannin sols (cp. pp. 129 and 174). 

Mechanical Operations. — In the tanyard the liquors are 
almost invariably divided up into sections, called "rounds " 
or " sets," in which the mechanical operations are different 
in aim and method. In the first pits entered by the goods 
there is rapid adsorption in spite of the low concentration and 
small astringency, and the great aim is to obtain evenness of 



VEGETABLE TANNAGE 47 

action and a good level colour. It is also necessary to main- 
tain the rate of adsorption. All the aims are attained by 
frequently moving the goods. Heavy leather is suspended 
vertically in the pits of tan liquor and handled up and down 
as well as forward from pit to pit. Such pits are termed 
" suspenders." In the earliest suspenders it is indeed advan- 
tageous to have the goods in constant motion. This is done 
by suspending on wooden frames which are rocked gently 
by mechanical power ; such pits are termed " rockers." 
For dressing leather in which firmness and smooth grain are 
not so essential, the goods may be paddled in the first 
liquors. This is occasionally done with stronger liquors for 
the express purpose of working up the " grain " pattern. The 
goods after passing through the suspenders are usually 
passed to " handler " rounds, in which they are moved less 
frequently. In these pits the goods are laid horizontally 
one above the other. One advantage of handlers is that 
the goods flatten thoroughly and straighten one another by 
their own weight ; another is that more goods can be placed 
in one pit than in suspenders. They are not so convenient 
to work, however, as suspenders, and the goods do not feed 
so rapidly. Hence the tendency is now to tan more in 
suspension, and to economize labour by an extension of 
the rockers. The handling of the goods is also saved by 
pumping the liquors and by working rounds of suspenders 
or rockers like the press leach system, with the difference 
that the stronger liquor is pumped in to the head pit, and 
the liquor passes upwards through the goods. 

Finally the goods are placed in " layers " or " layaways," 
in which they remain undisturbed for a decidedly longer 
time. These pits contain the strongest liquors of the yard, 
and their principal function is to complete the tannage and 
give weight and firmness by the adsorption of bloom, reds, 
etc., in the interior of the hide. The goods are placed in 
horizontally, and are dusted in between with fresh tanning 
material which maintains the local strength of the liquor 
and keeps the goods somewhat apart. Drum tanning attains 
a more rapid penetration of the pelt by giving constant 



48 ANIMAL PROTEINS 

motion in stronger infusions. It is of course liable to result 
in an under-tannage of the interior of the fibre. After the 
goods have been " struck through " in the ordinary way, 
however, drumming in extract is increasingly used as a 
substitute for much labour in handling, and also to save the 
time spent in the early layers. 



REFERENCES. 

Procter, "Principles of Leather Manufacture," pp. 220-350. 
Bennett, "Manufacture of Leather," pp. 113-179. 
Bennett, "Celavinia and Babla," L.T.R., 1914, 122. 
Dumesny and Noyer, "Manufacture of Tanning Extracts." 

Theory : — 

Meunier and Seyewetz, Collegium, 1908, 195. 

Stiasny, Collegium, 1908, 117-159, 289, 294, 337. 

Procter and Wilson, Collegium (London), 1917, 3. 

Wilson, Collegium (London), 1917, 97, 100, 105. 

Moeller, Collegium (London), 1917, 13, 38, 46, 103 ; and J.S.L.T.C, 1917, 
22, 56, 92. 

Bennett, J.S.L.T.C, 1917, 130-133, 169-182; 1918, 40; 1920,75-86; 
S.L.R., 1916, March. 



Section IV.— FINISHING PROCESSES 

After the tannage is complete, leather is hung up to dry. 
In the case of heavy leather this drying must be very 
carefully carried out in order to obtain a product of satis- 
factory appearance and saleable qualities. Associated with 
the drying are many mechanical operations (scouring and 
rolling) which assist very materially in imparting the desired 
qualities. After tanning, however, the quality of the final 
product is most strongly influenced by the amount of grease 
added in finishing. Some grease is always used in finishing, 
partly because even sole leather requires some measure of 
pliability and partly because a coating of oil over the leather 
during drying prevents the loose tannin from being drawn 
to the surface of the leather by capillarity, thereby causing 
dark and uneven patches and a " cracky " grain. The 
added grease is also a contribution to the " weight " of 
the finished article — a primary consideration for heavy 
leather, which is usually sold by weight. The finishing 
processes, indeed, tend to be dominated by this considera- 
tion, and become a series of efforts to retain as much tannin 
and add as much grease as are consistent with the require- 
ments of the class of leather being manufactured. Sole 
leather does not contain more than about 2 per cent, grease, 
or its firmness is impaired. Belting leather, in which 
considerable pliability is needed, may contain about 9 per 
cent., whilst harness leather, which must be exceedingly 
tough and durable, may contain up to 13 per cent, of fatty 
matters. Upper leathers, which need to be soft and pliable 
as well as waterproof and durable, are very heavily " stuffed" 
and often contain up to 30-40 per cent, of grease. Sole 
leather is thus rather distinct from the rest, which are called 
" curried," " stuffed," or " dressed " leathers, 
a. 4 



50 ANIMAL PROTEINS 

The actual drying out before, after and between the 
various mechanical operations, each have an appropriate 
degree of wetness. In this country the drying is usually 
under the prevailing atmospheric conditions and is known 
as " weather drying." The goods are suspended by 
hooks or strings or by laying over poles in special sheds 
fitted with louvre boards by which the rate of drying can 
be roughly controlled. Weather drying is cheap, but 
exceedingly slow, and in unfavourable weather is very un- 
reliable. The goods, moreover, need constant attention to 
obtain an even result. Steam pipes are usually laid along 
the shed floors, and are used in winter and damp weather to 
accelerate the drying, and also in the final shed stove to 
remove the last traces of moisture. Wet weather, however, 
will not stand a high temperature, and steam drying is 
better avoided when possible. Air-dried leather still con- 
tains about 14 per cent, of moisture. Many systems of shed 
ventilation have been suggested to hasten the drying and 
to secure a better control of the process. In one system a 
screw fan is fitted at one end of a shed (without louvre 
boards) and sucks air through the goods from an inlet at 
the other end. The air can be heated by a steam coil near 
the inlet. In another system a centrifugal fan blows air 
through an arrangement of pipes which distributes it to 
the drying sheds, and discharges it close to the floor by 
various branch pipes. The outlets are near the roof. A 
system of dampers permits hot air, warm air and the used 
wet air to be blended in the desired proportions. In America 
turret drying has been used. The sheds are vertically 
above one another and have latticed floors. Heated air is 
admitted at the bottom and rises through the goods up the 
building just as in a chimne}'. For many of the finishing 
operations it is important to obtain the leather in a uniformly 
half dry or " sammed " condition. This may be done by 
careful drying, and wetting back the parts that have 
become too dry with tepid water or weak sumac liquor, 
and then leaving the goods "in pile" until of uniform 
humidity. It may also be done by " wetting back " leather 



FINISHING PROCESSES 51 

which has been completely dried out. There are also 
" samming machines," which by means of rollers squeeze 
out the excess liquor. Sole leather is dried out and finished 
immediately after tanning, but dressing leather is often 
" rough dried " out of tan liquors and wet back for finishing 
when required. Dressing leather is often treated in different 
factories ; tanners selling it as rough leather and " curriers " 
finishing it. 

Scouring is one of the first operations in finishing leather. 
The grain side is wet and worked with brushes and stones 
until the bloom and loose tannin are removed. This pro- 
cess aims at producing a good even colour and level surface, 
but is liable to cause a loss of weight. Dressing leather is 
often scoured on both grain and flesh, and weak soap or 
borax solutions are used to assist the process. In this opera- 
tion hand labour has been now quite superseded by machine 
work. A great variety of machines have been devised. 
The mechanical working of leather takes place in various 
parts of finishing. These operations, known as " striking," 
" setting," " pinning," " jacking," may be carried out often 
by the same machine as used for scouring, but with a change 
of tool. The object of these operations is to get rid of 
wrinkles and creases, to produce softness, pliability and area, 
and to remove superfluous moisture, grease, dirt. The tools 
are of steel, brass, slate or vulcanite. Scouring is often 
effected by putting the goods into rotating drums together 
with extract and sumach. The bloom is removed by 
friction, the colour is improved by the sumach, whilst the 
extract keeps up the weight. 

In finishing sole leather firmness is enhanced by " rolling." 
A brass roller passes to and fro over the goods with the 
exertion of considerable pressure. The operation is carried 
out by machinery. 

Shaving is an important operation in the case of many 
dressing leathers. Its object is to produce a uniform 
thickness of the leather and an even surface on the flesh 
side. The sammed goods are laid over suitable beams 
and shaved with special sharp knives which possess a turned 



52 ANIMAL PROTEINS 

edge. This hand process, which demanded considerable 
skill, is fast becoming extinct, and machine shaving is 
already almost universal on account of its greater speed. 
The machines consist essentially of two rollers, one of which 
is smooth, whilst the other is a spiral knife-blade cylinder 
(cp. Section II., p. 23). The sammed goods are held in the " 
hands and placed over the smooth roller, which is raised 
to the cutting roller by a foot treadle. A number of similar 
operations (" flatting," " whitening," " buffing ") are 
carried out by a suitable change of tool. In all these 
operations good samming is important. 

Splitting is another important operation on tanned 
leather. In this process the leather is cut parallel to its 
grain surface, thus yielding two pieces with the same area 
as the original, the " grain " and the " flesh split." It is 
essentially a machine operation, and is carried out by 
presenting the carefully sammed leather to a sharp knife- 
edge, towards which it must be constantly pressed. The 
" band knife " machine is the most popular arrangement. 
The knife is an endless belt, which continually revolves 
round two pulley wheels of equal size. In between these 
the knife is horizontal, and is then used for splitting. The 
sammed leather is pushed towards the blade by two feed 
rollers, and the grain passes above the knife on to a small 
platform, whilst the flesh or " split " passes below and 
falls to the ground. Emery grinders and thick felt cleaners 
in the lower part of the machine keep the knife in good 
condition. The adjustment of the machine is delicate and 
requires considerable experience. With care splits may be 
obtained down to ^y thick, and sometimes as many as 
6 or 7 splits are obtained from one hide. 

Oiling is still usually done by hand, and cod oil is still 
preferred for many classes of goods. Of recent years there 
has been a great extension of the use of sulphonated oils, 
which have the valuable property of forming an emulsion 
with water or tan liquor. With these materials it is easier 
to ensure the goods being completely covered with oil. 
The penetration of the oil into the leather is also quicker 



FINISHING PROCESSES 53 

and more complete. These oils have often the disadvantage 
of leaving solid fats on the exterior of the leather, which 
gives it an ugly smeared appearance. 

Stuffing the dressing leathers is carried out in a variety 
of ways and with a variety of materials. The old process of 
hand stuffing employs a mixture of tallow and cod oil called 
1 ' dubbin. ' ' This is made by melting the ingredients together 
and allowing them to cool with constant stirring to a nearly 
homogeneous salve. The dubbin is brushed thickly on to 
the flesh side of the sammed leather, which is then hung up 
to dry. As the moisture dries out the oils and soft fats 
penetrate the leather and leave the more solid fats on the 
outside. The proportions of tallow and oil are varied with 
the time of year and with the method of drying, for 
if the dubbin be too soft it will run off the leather, and if 
too hard will not penetrate it so well. 

Drum stuffing is a more modern development in which 
a higher temperature is employed, about 140 F. The drum 
is heated up by steam or by hot air, and the sammed goods 
are then inserted and drummed for a few minutes until 
they are warmed. The drum is fitted with a heated funnel 
containing the melted grease, which is run in through the 
hollow axle. After a half to three-quarters of an hour's 
drumming the grease is completely absorbed by the leather. 
The drumming is continued for a while until the goods have 
cooled. Whilst still warm they are " set out " to remove 
creases and superfluous grease. Drum stuffing is not only 
quicker than hand stuffing, but also makes it possible to use 
the hard fats, and so make a leather which carries more grease 
without appearing greasy. Thus in drum stuffing, paraffin 
wax and wool fat are used, and their penetration assisted 
by small proportions of cod oil or degras. If the leather be 
too wet the grease is not absorbed, whilst if it be drier than 
usual the leather will take more grease, but the resulting 
colour is not so good. There is also another method of 
stuffing which originates from the Continent. It is known 
as " burning in," and involves the use of still higher tem- 
peratures (195 to 212 F.). Wet leather will, of course, not 



54 ANIMAL PROTEINS 

stand this temperature, so that it is first necessary to make 
the leather absolutely dry. This is effected by drying in 
stoves at temperatures up to iio°-ii5° F. There are two 
ways in which the grease is applied. In one method the 
melted grease is poured by a ladle on to the flesh side and 
brushed over until evenly distributed. A second applica- 
tion of grease is made to the thicker parts. The hides are 
then put into warm water (120 F.) for about a quarter of 
an hour, and then drummed for half an hour. In the other 
method the goods are completely immersed in the melted 
fats for a few minutes in a steam-jacketer tank at a tempera- 
ture of 195 F. After softening in water at 120 F. the 
goods are drummed. " Burning in " is used for the heavier 
dressing leathers such as belting and harness. It does not 
give good colour, but permits the employment of still more 
hard fats. 



REFERENCES. 

Procter, "Principles of Leather Manufacture," pp. 223, 378. 
Bennett, "Manufacture of Leather," pp. 251-312. 

Bennett, " Principles of Leather Stuffing," Leather Trades Review, 191 1, 
186: 



Section V.— SOLE LEATHER 

IyEATHER for the soles of boots and shoes is a matter of 
essential interest to all, and forms one of the best appreciated 
applications of animal proteids to useful purposes. Methods 
for its manufacture are as numerous as the factories producing 
it, hence all that can be done is to describe broadly the 
general method which is typical of our time, to classify the 
many varieties into types, and to indicate the recent changes 
and present tendencies. 

Sole leather is mainly manufactured from butt pelt, and 
the great aim is to produce a firm, thick, waterproof and 
smooth grained leather which will bend without cracking. 
It must have a light tan colour to be saleable, and contain 
as much weight as possible to be profitable. 

The modern mixed tannage of " sole butts "or " scoured 
bends " generally utilizes ox-hides of the Scotch and English 
markets, though salted Continentals and South Americans 
are also employed. After the usual soaking a short and 
sharp liming is given. The special aim in liming sole hides is 
to obtain the maximum plumping effect with the minimum 
loss of hide substance. Both these achievements are 
necessary to obtain good weight. The limes should be 
kept as clean as possible, which is best obtained by putting 
clean hides into work. This reduces bacterial activity and 
loss of hide substance. The " shortness " of the process is 
attained by the use of sodium sulphide (from 2 to 16 ozs. 
per hide of sulphide crystals), by which depilation may be 
accomplished easily in about nine days. The amount of 
sulphide should be increased somewhat in the short-hair 
season and in cold weather. Some factories take up to about 
12 days using less sulphide, whilst others will lime in about 
a week by using the larger quantities. The amount of lime 



56 ANIMAL PROTEINS 

used varies enormously, and is invariably in great excess 
of the actual requirements. " Probably 2-3 per cent, on 
the green weight of the hides is all that can be really utilized, 
the remainder being wasted." x This amounts to about, 
2| lbs. lime per hide, but in practice it is more frequent to 
find 7, 8, 9 or even 10 lbs. per hide being used. The excess 
is innocuous, owing to the limited solubility of lime. Some 
excess is desirable, to replace in the liquor the lime adsorbed 
by the goods in plumping, to assist bacterial activity (p. 21), 
and also because in sharp lime liquors the undissolved 
portions do not remain so long in suspension. The use of 
sulphide and other alkalies does not " make it possible " to 
reduce the amount of lime used, it merely renders the excess 
more superfluous. The use of sulphide not only shortens 
the process, but also sharpens it, on account of the caustic 
soda produced by hydrolysis. Usually for sole leather, 
however, it hardly sharpens it sufficiently, and it is very 
common to add also caustic soda (or carbonate of soda) 
to the limes. About 2 ozs. caustic soda (or its equivalent 
in carbonate) is used per hide. The hides are limed generally 
by the three-pit system, giving about three days in each pit. 
They should be handled each day in the first pit (old lime) 
and once in the other pits. 

Unhairing and fleshing by hand labour is still common, 
in order to avoid great pressure on the plumped hide. 
Scudding should be very light, and in some yards is entirely 
omitted. Only the lime on the surface of the hide should 
be removed by deliming, and this immediately prior to the 
insertion of the butts into the tan liquor. This is to ensure 
good colour and yet keep the butts plump. Boric acid is 
the best for this purpose, using 10-15 lt> s - per 100 butts. 
The goods are inserted (and preferably rocked) in a dilute 
solution for a few hours only. About the same quantity of 
commercial lactic acid may be substituted for the boracic. 
This deliming can also be accomplished by adding the acid 
to the worst suspender in the tanyard. 

To obtain firmness and plumping it is necessary that 
1 Procter, "Principles of Leather Manufacture," p. 129. 



SOLE LEATHER 57 

the early liquors in tanning should be more acid than for 
other leathers. With old methods of tanning one could 
trust to the natural sourness of the liquors to complete the 
deliming and replump the goods with acid. In such cases 
any deliming was also unnecessary. In the modern yard, 
however, we get " sweet " liquors coming down the yard, 
partly on account of the greater proportion of extract used 
and partly because the liquors themselves are not so old. 
Hence it is now practically always necessary to acidify 
artificially the tan liquors. This may be done by adding 
a few gallons of lactic, acetic, formic, or butyric acid to the 
handlers and suspenders, especially in the winter and spring. 
It is now increasingly common to place sole butts in a 
special acid bath after they have been in tan liquor for about 
a week. This bath is often made from sulphuric acid, and 
may be 1 or 2 or even 4 per cent, in strength. 

The actual tanning of sole butts lasts three to four months, 
and just prior to the war the tannage consisted often of about 
one-third myrabs, one-third valonia, and one-third extract. 
The myrabs and valonia were leached together, and the 
extract added to the best leach to make layer liquors of the 
required strength. Some mimosa bark was generally used 
also, and now it is extensively employed to replace the 
valonia. The most widely preferred extract is chestnut, 
but quebracho, myrabs extract and mixtures have also a 
prominent place, and mimosa bark extract an increasing 
importance. It is recognized that this tannage is if any- 
thing too mellow, and that if only a smooth grain and plump 
butt "an be ensured in the first weeks of tanning, it is much 
better for sole leather to employ the most astringent tans 
possible and the sharpest liquors (i.e. liquors with a small 
relative proportion of soluble non-tannin matters). Hence 
there is the tendency in sole-leather tanning to employ 
fresh clear liquors for the butts and use up the more mellow 
liquors on the " offal " (shoulders and bellies). 

Four types of sole butt tannage will now be described, 
all of which illustrate the methods employed in a modern 
mixed tannage. 



58 ANIMAL PROTEINS 

I. The first type consists in a four-months tannage, in 
which the liquors are worked down the yard. 

The butts pass first through the suspenders (20°-40° Bkr.) 
in about a week, and are rocked in the first liquors. They 
next enter the handlers (40°-55°) rounds of eight pits, six 
floaters and two dusters. Myrabs, or a mixture with 
algarobilla is used as dusting material. The goods remain 
in this set for two weeks, and should then be struck through. 
The suspender handlers (55°-65°) are next entered, in which 
they remain up to three weeks in suspension, being shifted 
forward on alternate days. The goods now enter the layers, 
of which four are given : first yo° for one week ; second 75 
for two weeks ; third 8o° for three weeks ; and fourth 90 
for a month. The goods thus take sixteen weeks to tan, 
of which ten weeks (62! per cent.) are in layers. 

The system of working the liquors is expensive, and is 
only possible if the butt liquors can be spent out by the 
offal. The best or fourth layer, 90 , is made from the best 
leach liquor, 65 , and extract (chestnut with some oakwood 
or mimosa bark). After use it becomes the second layer, 
75°. The third layer, 8o°, is also made from fresh leach 
liquor and extract (chestnut with some myrabs or mixed 
extract). After being used thus it is used for the first 
layer, 70 . The used first and second layers are mixed 
together and used partly to form the belly layers, and partly 
to make a sharp liquor for the handlers (55°-40 o ) by diluting 
with 40 ° leach liquor and adding quebracho extract. The 
old handler liquor is run to the suspenders (40°-20°), and 
finally used for colouring off the offal in drum or paddle 18 . 
The suspender handlers (65°-55°) are made from fresh 
leach liquor and chestnut extract. They are afterwards 
used to make shoulder layers. The course of the liquors is 
shown on p. 59. 

It will be seen that fresh leach liquor and fresh material 
are used to each set except the suspenders, which must 
have some mellowness to ensure plumping and smooth grain. 
Iyayer liquors are used twice only, and then (when only five 
weeks old) pass to the handlers. These are further sharpened 



SOLE LEATHER 



59 



by fresh leach liquor and fresh extract and dry materials. 
The forward handlers are fresh liquors with fresh extract. 
This tannage is fairly typical of high-class sole leather, in 
which the liquors are worked down the yard, but worked 
towards the offal, which thus receives liquors with relatively 
greater proportions of mellow tans and soluble non-tans. 



I^each liquor— 



4th layer, 90 
2nd layer, 75 



\i 



3rd layer, 3o° 
1st layer, 70 
(mixture) [-> bellies] 



Handlers, 
55° 40 

Suspenders [-> offal] 



Suspender handlers -> 
( 6 5°-55°) (shoulders) 



2. The second type consists in a tannage of about four 
months, in which the liquors are not worked down the butt 
yard. In this method also there is an attempt to save 
much of the labour in handling, first by shortening the time 
in the handlers by one week (as compared with the above) , 
and second by fusing the two progressive handler sets into 
two sets of equal strength, through which the goods pass 
more slowly and with less disturbance. 

The goods go through the suspenders (io°-25°) in about a 
week, rocking in the early liquors, and then into large rounds 
of handlers (30°-45°) for one month. The handlers consist 
of floaters and several dusters, in which the butts are laid 
away with 1-3 cwt. myrabs. The goods next enter the 
layers, of the same strength as in Type 1, and in which 
they remain the same time. The total tannage is thus 15 
weeks, of which 10 weeks (nearly 67 per cent.) are in 
layers. 

The best or fourth layer is made up from leach liquor 
and extract, and is then used successively as a third, second 
and first layer, and then passes to the offal layers. The 



60 ANIMAL PROTEINS 

handler liquor is made entirely from fresh leach liquor and 
quebracho extract, and is a sharp liquor of greater strength 
than its Bkr. strength would indicate. The old handler 
liquor is run to the butt suspenders. The course is repre- 
sented thus : — 

I^each liquor 

4 th layer, 90 

3rd layer, 8o° 

2nd layer, 75 

1st layer, 70 -> [offal layers] 
Handlers (45°-3o°) 
Suspenders (25°-io°) 

3. The third type consists of a short three-month's 
tannage in which the liquors are worked straight down the 
yard. To compensate for the short time it is necessary to 
have stronger layer liquors in which the goods spend a still 
greater proportion of their total time. The stronger liquors 
involve a greater proportion of extract, particularly of 
quebracho, which fact causes the whole of the liquors to be 
sharper than their Bkr. strength indicates, and justifies 
them being worked straight down the yard. 

The goods go through suspenders (20°-4o°) as usual one 
week, and then pass into suspender-handlers (40°-6o°) for 
two weeks, and thence to the layers. In the nrst two of these 
(65 and 70 ) they are actually in suspension, a week in 
each liquor. They are then dusted down for ten to eleven 
days, first in 85 and then in a 95 liquor, and finally for a 
month in a liquor of no . The total tannage is thus twelve 
weeks, of which nine weeks (75 per cent.) are in layers. 
There is considerably less handling than in Type 2, and 
it is more convenient, the goods being in suspension. 

4. The fourth type is also a three-month's tannage. In 
this it is attempted to obtain even greater weight with still 
less labour. The layer liquors are kept much stronger by 



SOLE LEATHER 61 

the more extensive use of extract, and this makes it imprac- 
ticable as well as too costly to run these liquors down the 
yard. They are therefore repeatedly strengthened with 
extract and used again. 

The goods go through suspenders (20°-40°) as usual 
one week, and then through a round of suspender-handlers 
(4o°-55°) consisting of fresh sharp liquor from the leaches 
together with quebracho extract. They are in this set two 
weeks, and then are laid away. They receive three layers : 
first, 105 ° for 2 weeks ; second, no° for three weeks; and 
finally, 120 for a month. Of the twelve weeks, therefore, 
nine weeks (75 per cent.) are spent in layers. In this 
method the goods are immersed in 3 per cent, sulphuric 
acid after passing through the suspenders. 

There is possible, of course, a tremendous number of 
variants of the above types. The number of handler 
rounds is determined by the number of butts being dealt 
with. With a large number it is more easily possible to 
arrange for them to be in progressive strength as in Type 1. 
There axe also many systems of working the layers, of 
which the most notable is to make the second or third layer 
from fresh leach liquor and extract, and strengthen it with 
extract for the succeeding layers. It is then used as a 
first layer and worked down the yard. 

The bellies and shoulders often go through separate 
sets of liquors, but it is common to put them through 
suspenders, and even handlers together. They receive, of 
course, a distinctly shorter tannage, and are often drummed 
with extract before laying away or after the first layer. 
By way of illustration, the course of the offal and their 
liquors may be given in the case of Type 1. The shoulders 
and bellies are coloured off in a paddle or drum with old 
butt suspender liquor, which is then quite exhausted. They 
then pass through suspenders (i8°-4o°) together in 4-5 
days, and go through a handler round (40°-55°) for 3 
weeks, including one duster. The bellies are removed after 
2 weeks, and given three layers (6o°, 70 , 8o°) of a week each. 
They receive, therefore, nearly 6 weeks in all. The shoulders 



62 



ANIMAL PROTEINS 



also have three layers (60 °, 65 ° and 80 °) of 2, 3 and 4 weeks 
respectively. 

The course of the liquors is shown thus : — 



Butt layets Butt suspender 

(and + extract) Handlers ( + extract) 

Belly layers Shoulder layers 

(8o°-6o°) (8o°-6o°) 



Offal handlers (55°-4<> ) 
Offal suspenders (40°-! 8°) 



Butt suspenders 



Offal drums 
(18 ) 



Drain 

The tanned butts are piled for 2-3 days, sometimes 
rinsed to remove dusting material, and then scoured either 
by machine or by drumming with sumac and extract. This 
removes bloom, but causes some loss of weight. " Vatting " 
or " bleaching " now follows, in which it is attempted not 
only to bleach the colour of the leather, but also to impart 
as much weight as possible. The vat liquor is made several 
degrees stronger than the last layer by means of quebracho 
bleaching extract and good coloured chestnut or myrabs 
extract. The liquor is kept warm by a steam coil, at about 
ioo° F., but not much more without risk. The goods 
remain in the bleach liquor 2-3 days and are then horsed or 
suspended to drain. Sumach is sometimes used in the vats. 
A new vat liquor must be made up after some weeks' use. 
The goods are sometimes rinsed in weak sumac liquor before 
vatting to get good penetration, and sometimes after to 
ensure good colour. 

The butts are next oiled and hung up in a dark shed 
and allowed to dry slowly and evenly to an " india-rubbery " 
consistency and rather slimy feel. They are then " struck 
out " by machine, wiped, re-oiled and again hung up to 



SOLE LEATHER 63 

dry, preferably with sulphonated oil. After a short drying 
to a suitable and even condition they are " rolled on," and, 
possibly after further drying, " rolled off " with greater 
pressure, and then dried for a day or two with the help of 
a little steam. Finally they are machine-brushed and sent 
to the warehouse, where they are weighed and classified. 

The offal is often drum oiled. It needs more striking 
and is, more difficult to obtain in suitable condition for 
striking, rolling. It is treated similarly to butts, but often 
also goes for dressing leather, and may be split. It is of 
some interest to compare the above processes with that 
once very popular manufacture of " bloomed butts " in 
the West of England from South American salted hides. 
These receive a liming from 12-14 days, using 12-16 lbs. 
of lime per hide. They receive then a tannage of about 
9 months, comprising 3 weeks in suspenders (20°-40°) — 
very sour and mellow liquors — 4 weeks in handlers (40°-55°), 
4 weeks in dusters (60 °) , 4 weeks in round made from hemlock 
extract (6o c ), and 20 weeks in six layers (6o°-90°) in which 
they were dusted heavily with valonia. Oakwood extract 
was used for the layers, which took 57 per cent, of the total 
time. The butts were scoured in a much-dried condition, 
so that only the loose and surface bloom was removed. No 
bleaching was given in the modern sense. 

In the old oak-bark tannage of sole leather up to 12 
months were taken for tanning, two-thirds to four-fifths of 
which time the goods were in layers. The strongest liquor 
rarely exceeded 50 even where valonia and gambier were 
also used, and rather more than 30 if not. 

It will be understood from the above that the tendency 
for many years has been to shorten the time and the labour 
required for tanning. Drum tanning is obviously the next 
stage in shortening the time. In one such process the butts 
are put through suspenders (25°-40°) for 2 weeks, drummed 
for 12 hours in an 8o° extract liquor, and finally in a neat extract 
200 for 36 hours. Drum tanned sole leather, however, is 
not as yet of good quality ; the grain is not smooth, and the 
heavy weight finish (striking and rolling) needed to counteract 



64 ANIMAL PROTEINS 

this tendency is liable to cause poor " substance." The 
leather, too, readily wets and goes out of shape. Possibly 
some drumming may be adopted to save time in the early 
layers, but the most serious rival to the 3 months' tannage 
is the waterproof chrome sole leather (Part III., Section V., 
P-I73)- 



REFERENCES. 

Parker, J.S.C.I., 1902, 839. 

Procter, " Principles of Leather Manufacture," p. 220. 
Bennett, "Manufacture of Leather," pp. 179, 259. 
Bennett, J.S.C.I., 1909, 1193. 



Section VI.— BELTING LEATHER 

The manufacture of belting leather is well illustrated by 
the tanning and finishing of " strap butts." In general, 
the tannage presents many points of great similarity with 
the tannage of sole leather ; indeed, the resemblance is so 
close that in some factories there is little difference observed, 
and the currying and finishing operations are relied on to 
produce the desired difference in final results. Nevertheless, 
there is considerable difference in the type and ideal of the 
two leathers, which may be expressed in trade parlance as 
a greater " mellowness " for the belting leather, and in the 
best methods of manufacture this fact is in evidence 
throughout the whole process of manufacture. 

In liming, there need be little difference between sole 
and belting hides, and a sharp treatment of 9-10 days, by 
the three-pit system, with a day or two extra in the coldest 
weather, would meet ordinary needs. For the conservation 
of hide substance and for the saving of time a shorter liming 
is sometimes given, in which more sulphide is employed 
than is usual for sole leather. Even the very short processes 
of liming, 1 to 3 days, which involve the use of strong 
solutions of sodium sulphide, have been successfully employed 
for belting leather. The tendency to harsh grain with such 
processes is not so serious a defect with belting as with sole 
leather, and can be minimized by careful deliming. American 
and Continental factories tend to favour the use of those 
quick processes which employ warm water in addition to 
sulphide. The hides after a short liming in sulphide limes 
are immersed in warm water, which greatly accelerates both 
the chemical and bacterial actions. For example, after 
about 3 days' liming, in which both old and new limes are 
K. 5 



66 ANIMAL PROTEINS 

used as usual, the hides may be thrown into water from 
ioo°-io5° F., and will be ready for depilation in 7 or 8 
hours. 

Even a stronger liming may be given, especially if the 
soaking is unusually prolonged. Such processes undoubtedly 
save hide substance, and the pelt is obtained more free from 
lime, but they have the disadvantage that the natural 
grease of the hide is only imperfectly " killed " (i.e. saponi- 
fied or emulsified), and may interfere with the normal 
course of the tannage. The plumping is also apt to be 
insufficient. On the other hand, liming processes are also 
used in which a mellower liming or a longer liming is preferred 
in order to produce the desired degree of softness and 
pliability in the finished leather. Belting must not be too 
soft, of course, and it will be clear that the required differ- 
ence from sole leather can be produced either in liming or 
tanning or partly in both. These considerations also 
decide whether bating is to be omitted or not. A hard 
astringent tannage in sour liquors after a sharp liming 
might make bating essential, but in these days it is usual 
to avoid it and produce the effect in other ways. A light 
bating of a few hours is sometimes given, but it is more 
unusual to delime the grain thoroughly with boric acid, 
using up to 20 lbs. per 100 butts. Crackiness is a fatal 
defect in strap butts, so that a sound grain must always be 
obtained. Generally speaking, therefore, strap butts receive 
more washing in water, and rather more deliming than sole 
leather, even when they are not bated. It is also usual to 
scud much more thoroughly, and to round a larger propor- 
tion of butt, especially in length. 

The tannage is usually carried out with a blend which 
includes a much greater proportion of the fruit tans, and 
correspondingly less of extract. 

Distinctly more myrabs are used than in sole leather 
tannages, in the dry material, and amongst the extracts 
chestnut is preferred to quebracho, and myrabs to mimosa 
bark, though all these may be used in some degree. In the 
past the most favoured extract has been undoubtedly 



BELTING LEATHER 67 

gambier, which gives a tannage which is easily curried and 
imparts the required mellowness to the uncurried leather. 
The great expense of this material, however, together with 
the advent of drum stuffing and shorter tannages in stronger 
liquors, have tended to cause a considerable reduction in 
the proportion used for strap butts, and to limit its employ- 
ment to the earlier stages of tanning. 

The same tendencies for reducing the time taken to 
tan, employing stronger liquors, and securing economy of 
labour in handling, have been evidenced in the tannage of 
strap butts as in sole butts. It is nevertheless true that, 
broadly speaking, strap butts receive rather more handling 
and rather weaker liquors than sole butts. A greater amount 
of mechanical assistance is also employed with early stages 
(paddling, drumming, rocking). This is less objectionable 
for curried leather than for sole butts. The handling is 
more usually in suspension. The liquors are usually worked 
straight down the yard as a greater mellowness is needed 
in the early liquors than for sole butts. The offal is given 
a separate tannage and often used for different purposes, 
e.g. the shoulders for welting and the bellies for fancy goods. 
Plumping with sulphuric acid is generally considered in- 
admissible for strap butts. It has been shown that leather 
containing sulphuric acid tends to perish after the lapse of a 
number of years. Sole leather will be worn up before this 
effect is observed, but belting is an article which is intended 
to last much longer, and the use of sulphuric acid is con- 
sequently inadvisable. Plumping must be obtained, to a 
considerable extent, but must be achieved by the organic 
acids (lactic, acetic, formic and butyric acids) . A few gallons 
of such acids are consequently added to the handlers, 
especially in the winter and spring. Less may be used in 
the autumn, when the layer liquors which fermented in the 
summer months have worked down to the suspenders. A 
mixture of these acids is usually better than any one alone, 
for they not only differ very considerably in price, but also 
have different powers of neutralizing lime and plumping the 
goods. Lactic acid (M.W. 90), Acetic acid (M.W. 60), and 



68 ANIMAL PROTEINS 

formic acid (M.W. 46) are each monobasic acids ; conse- 
quently 3 lbs. formic will neutralize as much lime as 4 lbs. 
acetic or 6 lbs. lactic. Their plumping powers are somewhat 
influenced by the anion. In determining what quantities 
to take, the commercial strength of the acids must also be 
considered. Formic is often 80-90 per cent, pure, acetic 
60-80 per cent., and lactic 40-60, but may be as low as 25 
per cent. The blend must be adjusted accordingly. As 
strap butts do not need the firmness of sole leather, less of 
these acids may be used than for sole butts. 

The exact nature of the tannage and the strength of the 
liquors is largely influenced by commercial considerations. 
If the manufacturer is both tanner and currier, he need not 
go to such great expense in strong liquors and in time in 
layers, for he can obtain some of this weight in currying. 
If, however, the tanner sells the butts rough dried, he 
must naturally aim at obtaining greater weight in 
tanning. 

The actual details of the tanning processes are as usual 
very varied, but may be classified according to type, just 
as in the case of sole butts. 

Illustrations will now be given. 

Type I, which may be compared with Type 1 for sole 
butts, is a tannage of about 5 months. The goods pass 
through suspenders (8°-30°) in 2\ weeks, and then pass to 
the handlers (30°-5o°), in which they remain a month ; they 
are then put into suspension again and pass through the 
suspender handlers (40°-55°), which takes 2 \ weeks. In 
this round much gambier is added, and the goods are 
frequently handled. Four layers are usually given, viz. 
first layer 55 °, one week; second layer 60°, two weeks; 
third layer 65 , four weeks ; and fourth layer 75 , four 
weeks. The tannage is thus 20 weeks, of which n weeks 
(55° P er cent.) are in layers. Extra layers may be given 
to heavier goods, using stronger liquors made up with extract. 
All liquors work straight down the yard. 

The tannage consists of 35 per cent, myrabs, 35 per 
cent, valonia, 10 per cent. Natal bark, and 20 per cent. 



BELTING LEATHER 69 

extract, chiefly gambier, though some chestnut and que- 
bracho are used. 

Type 2 represents the modern tendency to use stronger 
liquors and a shorter time. The strap butts pass through 
the suspenders (22°-5o°) in if weeks, during about a third 
of which time they are rocked. They next pass through 
two sets of suspender-handlers (50°-67° and 6y°-8o°), which 
takes a month, and thence to the layers. Three layers are 
given (85 , 90 and ioo°), in which the goods remain one, 
three and four weeks respectively. The tannage is thus 
13J weeks, of which 8 weeks (nearly 60 per cent.) are in 
layers. The liquors work down the yard. Longer time 
may be given to heavier goods. The tannage consists of 
40 per cent, myrabs, 35 per cent, valonia or Natal bark, 
and 25 per cent, extract, chiefly chestnut, though some 
gambier may be added to the suspenders. 

However tanned, strap butts are first dried out rough 
over poles. This assists in making the tannage permanent, 
on account of secondary changes discussed in Section III, 
p. 46. They are next wet back for currying by soaking 
in water or sumach liquor for a few hours and piling to 
become soft and even. The first operation is "skiving," 
which is a light shaving on the flesh side, carried out by 
a sharp slicker with a turned edge. The butts are next 
scoured thoroughly by machine on both flesh and grain, 
and sumached in a vat for several hours at ioo° F., after 
which they are slicked out and hung up in a cool shed to 
samm for stuffing. Hand stuffing is often still preferred, 
with tallow and cod oil. The butts are next set out, and it 
is important that this should be thoroughly done. Machines 
are now generally used, and the goods are often re-set after 
further drying. After drying out completely they are given 
a light coating of tallow and laid away till wanted for 
cutting up into straps, which is now done by machinery. 

A Continental method for making belting leather is to 
give 6 weeks in a suspender set (70°-24°) of twelve pits 
arranged on the press system, running two fresh liquors a 
week, and to give them two layers (24 and 28 ) of 6 and 



70 ANIMAL PROTEINS 

8 weeks. The material is chiefly pine bark, but some oak 
bark, valonia, myrabs and quebracho are also used. The 
goods are stuffed by " burning in," molten fat being poured 
on the flesh side. 



REFERENCE. 
Bennett, "Manufacture of Leather," pp. 194, 295. 



Section VII.— HARNESS LEATHER 

Whkn discussing the question of oak bark (Section III.), 
reasons were advanced for its decreased use and popularity. 
These were quickly appreciated in the sole leather trade, 
but the obsolescence of oak bark in the dressing-leather 
section was much more prolonged, partly because there was 
less pressing need to obtain good weight in the actual tanning, 
and partly because in some branches of dressing leather, 
such as belting and harness, a leather was required of great 
durability and toughness, for which qualities oak bark 
tannage had a deservedly high reputation. Hence harness 
leather manufacture affords a good illustration of the 
transition between the methods of the late nineteenth and 
those of the twentieth century. With the use of oak bark 
lingered the old methods of liming, bating and tanning in 
weak liquors for a long time with plenty of gambier. Hence 
in this section it will be necessary to observe a gradual 
transition of method, both in wet work and tanning. It 
should be pointed out that this transition has not been and 
is not going on in all factories at the same rate. Many 
factories remain in which the old methods are still preferred 
at some stages of the manufacture, and some remain in 
which many of the changes indicated below have not taken 
place at all. The leather trade has always been considered 
conservative in its methods, but it should be realized that 
much of the prejudice in favour of old methods is due to 
the public, and that after all tanners and curriers, like other 
business men, have to suit their customers. The march of 
industry is not like a regiment in line ; it is rather more like 
nature, a survival of the most adaptable. 

Hides for harness leather are limed in various ways, of 
which the following are types. 



72 ANIMAL PROTEINS 

1. A rather mellow liming of 10-15 days (longer than 
for sole leather), in which nothing bnt lime is used, and a 
certain amount of old liquor used in making up the new 
limes. The liming was carried out by the one-pit system, 
but the goods and liquors were kept clean by a good soaking 
process. Hence the loss of hide substances was not very 
great ; goods so treated were bated before tanning. 

2. A shorter liming than the above by the three-pit 
system. This saved time (taking 9-10 days), saved hide 
substance, and ensured greater regularity of treatment. 
The limes were about as mellow, but a little sulphide (2-4 ozs. 
per hide) was used to assist the depilation, especially during 
the short-hair season. These goods were also bated. 

3. A distinctly longer liming, 15-16 days, in mellower 
limes. This differed from Type 1 also in the respect that 
greater regularity was ensured by the three-pit system ; a 
foot or two of old liquor was used in making up the new 
lime. More hide substance was lost than in either of the 
above processes, but this was deliberate, the object being 
to dispense with bating, which is always light for harness 
hides. Thus a longer and mellower but systematic liming 
was used as a substitute for shorter liming and bating. No 
sulphide was used in this process. 

4. A short liming of 6-7 days, using up to 12 ozs. of 
sulphide per hide. The object here is to save time and hide 
substance. The three-pit system is preferred. Bating again 
becomes necessary, but the pigeon- dung bate is replaced 
by artificial bates, less objectionable, quicker, and more 
scientific in management. 

5. A still shorter process of about five days, using still 
more sulphide (about 16-20 ozs. per hide), together with 
some calcium chloride to reduce harshness. In such a 
method there is a tendency to revert to the one-pit system, 
which involves rather less labour. The three-pit system 
shows to a great advantage in the longer processes of 
liming when the process is reduced to five days; there is 
little difference between the two, for a one-pit system is a 
two-liquor method. Hence again an artificial bate is used. 



HARNESS LEATHER 73 

The various methods of liming, together with analogous 
variations in tannage, have resulted in great variety in 
bating. Sometimes up to three days' bating has been given 
at yo° F., but more often the goods are merely immersed 
overnight, and then delimed with boric acid, but with 
sulphide processes it is an advantage to use some of the 
commercial bates of the ammonium chloride type, and 
finish off with boric acid. Scudding is always more thorough 
than for sole or belting ; the hides are rounded into long butts 
which include most of the shoulder " harness backs." The 
goods are sometimes bate shaved. 

A few tannages will now be outlined, in order of historic 
type. 

Type I may be taken to represent the so-called 
" high-class " process in which oak bark myrabs and 
valonia are the staple materials. A good deal of gambier 
is also used, and a little myrabs and chestnut extract are 
helpful in attaining the desired strength of liquor. The 
"backs " go first through suspenders (8°-30°), which takes 
up to three weeks, and then in to handlers (30°-40°) for four 
weeks, consisting of rounds of clear liquor. They next go 
through a duster round, in which they are put for a week 
with oak bark and myrabs into a liquor of 45 . Four layers 
are given (50 , 55 , 6o° and 65 ), in which the goods remain 
for two, thre'e, four and five weeks respectively, oak bark 
being the chief dusting material. The tannage is thus for 
twenty weeks. Light backs receive less time in the layers 
(only 11 weeks). If the tanner is also the currier, the fourth 
layers are omitted. He then saves five weeks and gets the 
weight in the stuffing. 

Type 2 is a tannage in which oak bark and valonia are 
replaced by myrabs, mimosa bark and chestnut extract. 
It is therefore considerably cheaper and probably no less 
durable. Expense is also curtailed in handling. The 
harness backs go through suspenders (i6°-30°) in two weeks, 
handlers (30°-45°) in four weeks, and then receive four 
layers of the same strength as in Type 1, but only one, two, 
three and four weeks respectively. The last layer is omitted 



74 ANIMAL PROTEINS 

for light harness, and an extra layer of 75 is given if the tanner 
is not the cnrrier also. Thus the usual tannage is 16-20 
weeks, of which 10-14 weeks (63-73 per cent.) are in layers. 

Type 3 is a tannage which may consist of myrabs (55 per 
cent.), valonia or mimosa bark 25 per cent., and extract (26 
per cent.). The extract is chiefly quebracho, though some 
chestnut may be used. More valonia and less myrabs may 
be used if desired (and when possible), and myrabs extract 
will then replace quebracho and chestnut. The goods are 
coloured off in drums or paddles, and then pass through 
two sets of suspenders handlers (20°-55° and 55°-75)- 
They are handled up and down very frequently in the first 
set and rapidly pass into stronger liquors. The backs then 
receive three floaters at 80 °, in each of which they remain 
one week. The tannage is completed by three layers : first, 
85 for one week ; second, 90 for one week ; third, 95 for 
two weeks. The tannage is thus n weeks, of which 7 weeks 
involve little labour. If the tanner is not the currier, still 
stronger liquors may be used. 

In all these tannages little or no acid is used for plumping, 
as the natural acids of the liquors are sufficient to ensure 
what is necessary in this direction for this class of leather. 
A little organic acid or even boric acid may be used in the 
earliest liquors for deliming purposes, when necessary. 
After tanning the goods are dried out and sorted in the 
rough state. Harness is a somewhat broad term, and 
there is scope for considerable variety in classification. The 
hides are sometimes not rounded until after tanning. The 
finished article may be any grade between heavy harness 
for artillery and leather for ordinary bridles. 

In currying heavy black harness, the backs are soaked 
and sammed for shaving. lighter goods may be machine- 
shaved, but the heaviest are shaved lightly by hand over 
the beam or merely " skived " with the shaving slickers. 
The neck needs most attention, and it is often advisable to 
stone by machine and split. The scouring should be 
thorough, on flesh and grain. This is done by machine, and 
not only cleans the goods from bloom, dirt and superfluous 



HARNESS LEATHER 75 

tan, but also assists in setting out. Sumaching may be 
for several days, merely overnight or even only for a few 
hours, being stoned after wetting back to temper. Hand- 
stuffed goods get a coat of cod oil first, and during the drying 
are often well set out. Drum-stuffed goods are well set 
out by machine, and after some drying, stoned and reset 
by hand. It is now usual to buff the grain, i.e. remove the 
coarser parts by light shaving. This prevents cracking in 
the finished article. The goods are blacked with logwood, 
iron and ammonia, thinly dubbined again, again well set 
out and tallowed. Setting out, indeed, may be done at 
any convenient opportunity. The superfluous grease is 
removed by slicking, scraping, brushing with a stiff brush, 
and finally with a soft brush. 

For brown harness the goods are more carefully selected, 
more thoroughly scoured and sumached, and bleached 
frequently with oxalic acid. They are hand stuffed, stained 
twice, and after the usual setting out, glassing and brushing, 
are finally rubbed with flannel. 

For bridle leather the goods are carefully shaved but 
are not stuffed, being merely oiled with cod oil on flesh and 
grain. They are dried out before scouring, and then sized, 
set out, stained and resized. The goods are heavily glassed 
during the finishing. 



REFERENCE. 
Bennett, "Manufacture of Leather," pp. 195, 297. 



Section VIII.— UPPER LEATHERS 

The manufacture of leather for the uppers of boots and 
shoes embraces a bewildering variety of goods, suitable for 
anything between a baby's shoe and a man's shooting boot. 
Almost all degrees of lightness, softness, and waterproofness 
are in demand. A great variety of finish is also involved, 
determined by the ingenuity of the currier and the ever- 
changing fancy of the public. Even greater is the variety of 
methods by which all these results are obtained by methods 
which superficially seem quite different ; the desired 
qualities being imparted in one case largely by the tannage 
and in another case almost entirely by the currying. Under 
such circumstances the selection of types becomes a 
problem. 

The variety, moreover, commences from the earliest 
stages, the selection of the raw material. Upper leather 
may be made from light calfskins, heavy calfskins, kips 
(home and foreign), light dressing hides and heavy dressing 
hides, which last may replace any of the former after 
splitting to the required substance. In this section it will 
be necessary to take kips as typical of the rest, and to use 
it in a rather broad sense, including heavy calf and light 
dressing hides. 

Speaking quite generally, kips for upper leather receive 
usually a long and mellow liming, a thorough bating and a 
sweet and very mellow tannage in weak liquors. In curry- 
ing they are well scoured and set out, heavily stuffed and 
stained black, being sometimes finished on the grain and 
sometimes on the flesh. These outstanding features of 
upper-leather methods will be further illustrated by a brief 



UPPER LEATHERS 77 

account of the tanning of kips (light hide and heavy 
calf), and outlining the best known types of finish for butt, 
shoulder and belly. 

The goods receive usually a long and mellow liming of 
14-16 days, using only lime as a rule. In some factories 
lime liquors are used repeatedly for successive packs to an 
almost indefinite extent. Dissolved hide substance, ammonia, 
mud and dust, and bacteria accumulate for months and 
sometimes for years. It is obvious that in such liquors 
"putrefaction" is a more correct term than "liming" for 
the depilation. Such methods have been used even in recent 
years, but there has now been a tendency for some time to 
make the liming more methodical. Such old limes make a 
leather which is empty, loose, and dull grained, but the 
defects are minimized by the system of stuffing heavily and 
finishing the flesh, and hence the ancient lime remained 
with surprising tenacity. Even so late as 1903 we find that 
Procter with characteristic caution could write, " Probably 
no lime ought to be allowed to go for more than three months 
at the outside limit without at least a partial change of 
liquor." It is within the writer's experience to find an upper 
leather factory with limes which had never been emptied 
for over three years. In other factories, however, there 
has been a revulsion of feeling with regard to such processes, 
and it has been found advantageous to adopt a more scientific 
routine, in which the lime pits are cleaned out at regular 
intervals. There is little doubt that a mellow liming is 
desirable, but this can be secured by blending some old 
lime liquors with fresh lime liquor in a systematic manner. 
Similar considerations apply to the question of working the 
various packs through the limes. It is clear that with a 
mellow liming a one-pit system is quite possibly satis- 
factory, but the revulsion of feeling against a lack of method 
produced a method of liming more elaborate than usual, and 
it is now not uncommon to find kips limed in a " round " 
of 6-8 pits, the goods passing through each pit. They 
remain in one pit about two days, and are shifted forward. 
In the green or old limes the goods are handled up and 



78 ANIMAL PROTEINS 

down. The old limes are, of course, mellower than the new 
and exert the desired softening effect. The working is 
quite analogous to that of a round of handlers. Unhairing 
is sometimes assisted by the use of arsenic sulphide. E.I. 
kips need a thorough soaking before any liming ; several 
days are usually needed. The old methods involving 
putrid soaks and stocks may be considered out of date, and 
it is usual to soften back in caustic soda or sulphide soaks 
with some assistance by drumming. A little sulphide is 
sometimes added to the older limes to continue the treat- 
ment. 

The goods are next thoroughly bated and delimed. The 
hen or pigeon dung bate is still usual, and probably gives 
the best results, though closer approximations have been 
made of recent years on artificial lines. Some bating with 
solution of hide substance seems necessary for these goods. 
The lighter goods are often drenched also to complete the 
deliming, using 6 per cent, bran on the weight of pelt. The 
heavier goods are more often treated with boric acid after 
bating, which not only delimes completely and gives a soft 
relaxed felt, but also acts as antiseptic and stops the action 
of the bate, a matter of some importance (see Section II.). 
Lactic acid may substitute boric, in which case about 
2 per cent, on the pelt weight of 50 per cent, acid 
may be required. It is important to avoid a strong 
solution and local excess, hence lactic acid must be added 
gradually so that the liquor is never stronger than o*2 
per cent. Drumming and paddling is an advantage in 
deliming . 

The tannage is light in most cases, partly because some 
of the finished goods are sold by area, but partly also 
because even if sold by weight, the weight is obtained 
quicker and more easily by stuffing, which course is also 
often preferable to obtain the desired mellow feel, water- 
proofness and durability. Hence it is seldom that strong 
liquors are employed. The tannage is also mellow, on ac- 
count of the softness and pliability required ; no acids are 
consequently employed, and no material which is liable to 



UPPER LEATHERS 79 

yield sour liquors. Gambier is easily the first favourite 
amongst the tanning materials, whilst oak bark comes 
second. It should be observed, however, that a hypo- 
thetical tannage of equal weights of cube gambier and oak 
bark is in reality a tannage by four-fifths gambier and one- 
fifth oak bark, on account of the relatively greater strength 
of the former. This observation is so apposite with respect 
to some tannages that it is nearly correct to say that the 
tannage is gambier and the oak bark an excuse for having 
leaches through which the gambier liquors may be run 
occasionally to clear and to sharpen slightly. No serious 
theoretical objection to such a method is possible if the 
liquors are weak and the system of working the liquors is 
scientific and the process carefully regulated. Upper-leather 
tannages, however, have scarcely merited scientific praise. 
It is often a case, not of poor methods, but of no method at 
all. The same lack of system, principle, and regularity 
observed with regard to the limeyard has been equally 
obvious in the tanyard, when perhaps the need was even 
greater. Even a mellow tannage has varying degrees of 
mellowness possible to it ; there still remains the question of 
the soluble non-tans. However, method in the upper-leather 
tanyard has often been conspicuously absent. There has 
been many a factory where any one tan liquor was as good 
as any other in the yard. In the writer's experience are 
two such cases : in one the liquors were all 25 Bkr., in the 
other they were all o° Bkr. In such cases, handling the 
goods from pit to pit is somewhat futile, and handling forward 
from set to set still more so. Hence it is possible to find 
dressing leather tanned by putting it slowly through one 
round of handlers, adding a few buckets of gambier where 
it apparently is necessary. It is, from one point of view, 
surprising to see what serviceable and excellent-looking 
upper leather can De manufactured by such happy-go- 
lucky processes. It is, however, also possible to see how 
this may occur. Gambier is a stable tan, and no souring 
and little decomposition take place in gambier liquors. 
It is also extremely mild and non-astringent, and is always 



80 ANIMAL PROTEINS 

used in weak liquors. The hides, moreover, are completely 
delimed, and there is little danger of bad or uneven colour. 
Tanning under these conditions is at its easiest ; it is almost 
more difficult to spoil the goods than make them right. 
Under such conditions tanning deteriorated rather than 
improved in method. When neglecting it made little 
difference to the finished leather, it was neglected. 

This state of affairs, however, was embarrassing when- 
ever a tanner wished to try any other tanning material. 
The expense of gambier and oak bark made valonia and 
mimosa bark into obviously desirable alternatives and 
substitutes. Methods which would tan with gambier, 
however, would not work with Natal bark or valonia, and 
many a tanner has had to revise his method of tanning 
from end to end. The use of myrabs also raised the problem 
of souring, and it has become evident that " working the 
liquors down the yard " is as desirable a method for dressing 
leather as after all other tannages. It will be clear from the 
above that types of upper-leather tannages are less typical 
than for other leathers, but nevertheless the more progressive 
manufacturers have for some years now been working 
on sounder lines, economically and scientifically. In 
such cases it is now usual to pass the goods through at 
least two sets of handlers, and through liquors of gradually 
increasing strength. Occasionally dusters or layers are 
given, especially for the heavier goods. The tannage 
is nearly always commenced now by paddling the goods in 
the oldest liquor. This paddling may be anything from 
half an hour up to twenty-four hours. It is sometimes 
desired to work up a " grain," and the old liquor is then 
often sharpened by the addition of fresh gambier or leach 
liquor. 

The same tendency to save labour in handling is to be 
observed in upper leather tannages as in sole and other 
dressing leather factories. There is also a tendency to obtain 
rather more weight in tanning by using stronger liquors, 
and in the heavier goods to shorten somewhat the time taken. 
The following methods may be taken to illustrate modern 



UPPER LEATHERS 81 

processes, in order of evolution. They all last about seven 
weeks. 

Type i. — In this process the kips are first paddled in an 
old liquor (3 ), and passed to the first handlers (3°-30°) 
for three weeks. After working through this set they pass 
through the second handlers (20°-30°), in which they are 
not handled quite so frequently. They are in this set also 
three weeks. Heavy goods may then receive a floater (30 ) 
for another week. 

Type 2. — In this process the goods are paddled, 
and then enter a large handler round (8°-30°), through 
which they pass in five weeks. The goods are handled 
frequently in the early stages. The tannage is com- 
pleted by one layer of two weeks (30°). The layer is made 
by the ancient method of putting the goods and dust 
alternately into an empty pit, and then filling up with 
liquor from the best leach. Oak bark, valonia and 
myrabs are used as dust, though sumach and gambier 
have been used. 

Type 3. — In this process an attempt is made to 
save handling and obtain more complete tannage. The 
goods are paddled for three to five hours in a rather 
sharp liquor of io°, and are then handled well for a 
week in the first handlers (5°-20°). The goods then go 
through the second handlers (20°-45°) in six weeks, and 
heavy goods may then receive an extra floater (45 °) for 
one week. 

In type 1 the leaching material is two-thirds oak bark 
and one-third valonia ; in type 2 it is half oak bark and 
half mimosa bark ; in type 3 it is one-third oak bark, one- 
third valonia or Natal bark, and one-third myrabolans. 
In all cases the strongest handler is obtained from the 
leaches, and made up to the required strength with strong 
infusion of gambier. When the liquor has passed through 
the forward handlers, it is returned to the leaches to clear 
and sharpen, and then run to the green handlers. After 
passing through this round it is again returned to the 
paddle, from which it passes to the drain. The rest of the 
E. 6 



82 ANIMAL PROTEINS 

paddle liquor may be from the forward handlers. It is 
often customary to obtain the best liquor from the second 
leach, and allow the best leach to stand for a few days. 
This allows the bloom to deposit in the leaches. The system 
secures the result desired, but the deposition of bloom 
involves a loss of tannin, which waste makes the system 
expensive. 

Heavier dressing hides are tanned by methods similar 
to the above, but with floaters, dusters and occasionally 
layers added after they have passed through two sets of 
handlers. Thus they may have first handlers (8°-i8°) two 
weeks ; second handlers (40°-45°) for six weeks, making 
twelve weeks in all. Lighter goods may receive two 
rounds, being two weeks in each. 

After tanning, the kips are rounded usually into 
butts, shoulders and bellies, to which different finishes 
are given. The currying may be illustrated by selecting 
types, but it must be borne in mind that there is much 
elasticity in this matter. Thus kips may be made into 
waxed butts, satin shoulders and lining bellies, but also 
may be cut down the back in " sides," both of which are 
finished limings. 

Waxed kip butts are a type of many similar upper 
leathers (waxed shoe butts, waxed calf, waxed splits, 
etc.). The finish is on the flesh side. The kip butts 
are soaked carefully, and shaved by machine. They are 
then drummed in sumach for an hour or two, slicked out 
and sammed for stuffing. The sumaching is also the 
scouring unless the goods be too heavily bloomed. The 
samming is often done by machine. Drum stuffing follows, 
wool fat and stearin being staple greases, with varying 
amounts of degras and cod oil, and of tallow and cod oil. 
A little paraffin wax and resin are also used sometimes. 
The goods are well slicked out and dried. They may be 
now dubbined and laid away to mellow for whitening, 
which consists of a careful shaving of the flesh by a turned- 
edge slicker or by machine. The grain is stoned, set out 
and " starched," and the butts grained by boarding the 



UPPER LEATHERS 83 

flesh. In the waxing, one of two courses may be adopted. 
The butts may be blacked with lampblack and oil, 
" bottom sized " with glue, soap and logwood, and then 
" top sized " with glue, dubbin, beeswax and turpentine ; 
or they may be given a " soap-blacking " of soap and log- 
wood and lampblack, applied by machine, and sized once 
only. 

Dressing hide butts may also be given a grain finish, 
such as the " memel butts " for heavy uppers. The butts 
are soaked, shaved or split, sumached in drum, and pre- 
ferably thoroughly scoured on flesh and grain. They are 
then sammed and heavily stuffed in the drum. The grain 
is buffed and stained black with logwood, ammonia and iron 
solution (curriers' ink). The butts are then dried, set out, 
thinly sized and slowly dried. 

When dry on the face they are printed or embossed 
by machine to give the characteristic memel pattern 
and dried out completely. They are then grained four 
ways. The grain is finished by a coating of linseed oil 
containing resin, and the flesh is whitened, French chalked 
and glassed. 

Shoulders for " satin " receive a currying which strongly 
resembles the " waxed " finishes, but the smooth finish is 
on the grain side. The grain is buffed, and blacked, 
dubbined, set and reset, with intermediate drying, and is 
sized and finished by compositions similar to those used for 
waxed leathers. The flesh is whitened. Satin hide and 
satin calf are dressed similarly. 

Shoulders may also be finished for " levant." After 
soaking, splitting, and shaving to substance, they are 
drum-sumached, machine-sammed, and oiled up to 
dry. They are stained with logwood on the grain, and at 
once printed with the typical " levant grain," blacked 
and dried out. They are then softened by machine, 
seasoned with logwood and albumen, glazed, grained and 
oiled lightly with mineral oil. It will be observed that 
stuffing is omitted. 

Bellies may be dressed for linings. After soaking and 



84 ANIMAL PROTEINS 

splitting to the required substance, they are bleached in a 
weak and warm solution of oxalic acid, and drum-sumached 
at no° F. After slicking well out they are hand-stuffed on 
the grain with dubbin and water, or merely oiled, and hung 
up to samm. They are then set-out flesh and grain. If 
the grain be coarse, it is buffed and reset. After drying 
out the flesh is fluffed and the grain dusted with French 
chalk. 

In this section may be conveniently discussed the 
manufacture of legging leather. 'Whilst in many respects 
a typical dressing leather there are some rather important 
differences from the average upper leather. Broadly 
speaking, the differences are that legging leather needs a 
smooth grain, greater firmness and more thorough tannage 
on account of the absence of stuffing. 

The liming and bating are somewhat similar to dressing 
leather, though a shorter liming with sulphides and a 
milder bating would be in order. The tannage is mellow, 
but not so much as is usual for upper leather. Thus gam bier 
is used, but more valonia and myrabs are employed, and 
the liquors may be strengthened with chestnut and que- 
bracho extracts. The hides are rounded before tanning 
into long butts or backs, and the tannage is commenced in 
suspenders (i8°-40°), which are kept acid by the addition of 
lactic or acetic acid, in order to obtain the required firmness ; 
the goods are three weeks in these liquors. The backs next 
go through rounds of dusters (40°-5o°), in which they are put 
down with oak bark and Natal bark. They are six weeks 
in this section, and then pass to the layers. Three layers are 
given, first 50 for one week; second 5 5° for two weeks; 
and third 6o° for two weeks. The tannage thus takes 
fourteen weeks. 

In finishing, the goods are soaked and split, and then 
scoured flesh and grain. They are heavily sumached, 
slicked out thoroughly, oiled up with linseed oil and dried 
out. They are then next damped back, stoned and flatted. 
After further wetting and tempering they are dressed with 
Irish moss and tallow on the flesh, and with gum tragacanth 



UPPER LEATHERS 85 

on the grain. They are glassed whilst drying out, and then 
stained twice and glassed again. They are again brushed, 
seasoned and glassed by machine. 



REFERENCE. 
Bennett, "Manufacture of Leather," pp. 197-201 and 301-308. 



Section IX.— BAG LEATHER 

Hides to be tanned for bag leather receive a treatment 
which is little different in fundamental principle from that 
of dressing hides for upper leather, except that the tannage 
is usually shorter. Hides for bags and portmanteaux 
represent a type of dressing leather in which the outstanding 
features are that the goods are split but not rounded. The 
splitting is done at all stages, according to the requirements 
of the tanner. Some tanners split " green," i.e. split the 
pelt itself. The advantage of this is that the fleshes may 
then be treated in quite a different way, e.g. pickled or 
given a much cheaper tannage. Other manufacturers split 
after tanning, the advantage being that there is much less 
material to handle. The general opinion, however, favours 
a middle course in which the hides are split after being in 
the tan liquors for a short time. The advantage of this 
course is that the hides are easiest to split under these 
conditions — a great consideration — being coloured through 
with tan, just a little plumped, but not hard. A smoother 
flesh is obtained together with more even substance. Here 
again, however, are differences; some tanners prefer to split 
after two days, others after two weeks in tan. Much depends 
upon the nature of the tan and the strength of the liquors. 

For this class of work, flat, spready and evenly grown 
cowhides are obviously the most suitable material, and are 
invariably used. It is important, however, that the grain 
be good, and free from scratches and similar defects. The 
tannage must be sweet and mellow, i.e. contain no acid and 
little astringent tan. Hence myrabolans and gambier have 
always been the favourite tanning materials. A soft and 
mellowtannage is the more important, inasmuch as theleathei 
is not heavily stuffed with grease in finishing. These types 



BAG LEATHER 87 

of method for tanning split hides will now be outlined, and 
the nature of the currying then indicated. 

Type 1. — In this a long mellow liming of 15-16 days is 
given, much like that described for harness leather in Section 
III., p. 72, Type 3. Only lime is used, but the liquors are 
not allowed to get dirty. The three-pit system is much the 
best. The hides are trimmed at the rounding tables, and 
then bated in hen or pigeon dung for three days at 75°-85° F. 
The deliming is commenced by washing in tepid water before 
bating, and is completed by a bath of boric acid, using up 
to 30 lbs. acid per 100 hides as necessary. In this and other 
processes for split hides it is essential to obtain all the lime 
out, but to do no plumping with acid. Lactic acid may also 
be used, but it is not so convenient to hit the neutral point 
with it. 

The tannage consists of oak bark and myrabs together 
with gambier. These may be partly replaced by Natal 
bark, valonia, and quebracho respectively. It is sometimes 
desired to have a smooth finish, but sometimes to work up 
a " grain." In the latter case the hides are first put through 
colouring pits containing fresh leach liquor. In these they 
are constantly handled for a few hours. A little experience 
indicates which leach liquor will serve the purpose. The 
hides then go through the " green handlers " (8°-20°) in 
two weeks. The liquor is the old forward handler liquor 
made up with gambier. The hides may be sammed and 
split up at this stage, but the heavier goods may be tanned 
further. These heavies and the grains of the split hides 
now go through the " forward handlers " (20°-40°) for four 
weeks, and the heaviest goods given two layers (40 °) of two 
weeks each, and making ten in all. 

Type 2. — In this a shorter liming of 8-9 days is given 
with the help of sulphide. No dung bate is used, but the 
goods are washed with water and bated with ammonium 
chloride and boric acid. The tannage is chiefly of myrabs, 
but some valonia or Natal bark may be used together with 
chestnut extract and some quebracho. Gambier is used in 
the early liquors. The goods are coloured off in drum or 



88 ANIMAL PROTEINS 

paddle and tanned in several sets of handlers, viz. green 
handlers (i5°-35°) three or four days ; second handlers 
(35°-6o°) two weeks ; forward handlers (6o°-8o°) i| weeks ; 
and floaters (8o°-go°) for three weeks. The tannage is thus 
6| weeks in all. The arrangement of pits is a matter of local 
convenience, and the number of sets of equal strength is 
determined by the number of hides being tanned. The 
hides are split green or after passing through the green set. 
After tanning they are oiled with cod oil and dried out. 

Type 3 is illustrated by American methods. The 
goods are tacked on laths or racks with copper nails in 
order to ensure smooth grain. They are then suspended in 
tan liquors. The tannage is largely with gambier and in 
weak liquors, which also help to give smooth grain. The 
tendency is to employ handler rounds involving a rather 
large number of pits, and to work these on the press system. 
Handling is also saved by plumping the liquors instead of 
shifting the goods forward, and by rocking the suspenders 
instead of handling up and down. The hides are split after 
about a month, and the heavier grains laid away in hemlock 
liquors. 

Type 4. — This is a rapid process throughout. The hides 
are limed in 6-7 days with the help of sulphide, and " bated " 
by washing in warm water and then in cold to which hydro- 
chloric acid is gradually added, finishing off again in tepid 
water. The hides are now coloured off in paddles, put 
through a small handler round (n°-20°) for half a week, and 
then split. The grains are drum tanned in a mixture of 
chestnut and quebracho extract, over a period of about 
three days in which the liquor is strengthened gradually 
from 30 to 50 . The fleshes are drum tanned with the old 
grain liquors after strengthening with quebracho. 

The split hide grains for bag work, after tanning, are 
drummed in sumach, rinsed, drained, and oiled up to dry 
out, with some setting out. After wetting back they are 
shaved if necessary, hand scoured, and heavily sumached 
again to get a light even colour. The goods are slicked out, 
oiled up to samm, reset and dried out. They are next 



BAG LEATHER 89 

stained, sammed, printed by machine, dubbined or tallowed, 
" grained " (see Part II., Section I., p. 97), brushed and 
rubbed with flannel. 



REFERENCE. 
Bennett, "Manufacture of Leather," pp. 202, 308. 



Section X.— PICKING BAND BUTTS 

It is the paradox of vegetable tannage that the less the 
pelt is tanned the stronger is the leather produced. The 
manufacture of butts for picking bands affords a good 
illustration. What is required is a leather of maximum 
toughness, pliability and durability. Any factor reducing 
the tensile strength of the leather is fatal. Hence, compared 
with most other tannages, picking band butts are under- 
tanned. To ensure the desired softness and pliability, 
moreover, it is necessary to have a mellow liming, rather 
heavy bating, and a soft mellow tannage in sweet and weak 
liquors. The required durability and the necessity for weak 
liquors both point to oak bark as the most suitable tanning 
material, assisted by some gambier in the early stages. 

A good quality hide is chosen, and given a long and 
mellow liming of about 15-16 days. The one-pit system 
may be used, and the hides are put into an old lime for about 
five days with frequent handling and then placed in a new 
lime which is made up in a pit containing about a foot 
depth of the old liquor. After about twelve days another 
2 cwt. of lime may be added. 

After unhairing and fleshing the goods are bated in 
pigeon dung for four days at a temperature of about 78 F., 
handling twice on the first and last days. The bating is 
stopped and the deliming completed by paddling with 
boracic acid (15 lbs. per 100 butts). 

The tannage is commenced by paddling in a spent 
handler liquor (4 ) to which a little gambier has been added. 
The butts then go through the first handlers (5°-i5°), 
which are rounds of ten pits in which the goods are handled 
every day in the first week, and alternate days in the second 
week, and are shifted forward twice a week in the next pit. 
The goods are therefore in this set for five weeks, Gambier is 



PICKING BAND BUTTS 91 

added to these liquors as needed. The butts next pass to 
duster rounds of four pits, in which they are dusted down 
in a liquor of 20 for four weeks with 1-2 cwt. of oak bark. 
The liquor is obtained from the leaches, and afterwards run 
alternately to the leaches and to the first handlers. As 
many as six layers are now given of 20°-25° strength, in 
which the butts are dusted down with 2-3 cwt. oak bark 
for three weeks. The layer liquors are received from and 
returned to the leaches, which are made from the " fishings " 
from the layers. The tannage lasts, therefore, 27 weeks, of 
which 18 weeks (two-thirds) are in layers. 

Shorter tannages are now often given, using stronger 
liquors, much as in ordinary dressing leather. 

The tanned butts are rough dried, and then wet in for 
shaving. They are thoroughly scoured, flesh and grain. 
They are next drummed for three-quarters of an hour in 
sumach, struck out and hung up to samm. Hand stuffing 
is best, to avoid any tendering owing to high temperature, but 
drum stuffing is also used. After setting out and stoning 
on the grain they are stuffed with warm cod oil and laid 
away in grease for several weeks, re-oiling occasionally. 
They may be stained before stuffing. 



REFERENCE. 
Bennett, "Manufacture of Leather," pp. 203, 310. 



Part II.— SKINS FOR LIGHT LEATHERS 

Section I.— PRINCIPLES AND GENERAL 
METHODS OF LIGHT LEATHER MANU- 
FACTURE 

The term "skin," like the term "hide," in its widest sense 
applies to the natural covering for the body of any animal, 
but is generally used with a narrower meaning in which 
it applies only to the covering of the smaller animals. Thus 
we speak of sheep skins, goat skins, seal skins, pig skins, 
deer skins, and porpoise skins. It is in this sense that it 
will be used in this volume. The treatment of such skins to 
fit them for useful purposes comprises the light leather trade. 
Whilst this branch of the leather industry is certainly 
utilitarian, the artistic element is a great deal more promi- 
nent in it than in the heavy leather branch. Thus the 
light leathers are often dyed and artistically finished, and 
their final purposes (such as fancy goods, upholstery, book- 
binding, slippers, etc.) have rather more of the element of 
luxury than of essential utility. The total weight and value 
of the skins prepared, and of the materials used in their 
preparation, are naturally considerably smaller than those 
of the heavy leather trade. In the latter, moreover, one 
has to consider the purpose in view from the very com- 
mencement of manufacture and vary the process accord- 
ingly, but in light leather manufacture one aims rather, in 
the factory, at a type of leather such as morocco leather, 
and only after manufacture is it fitted to such purposes as 
may be particularly suited to the actual result. These 
results depend very largely upon the " grain pattern " 
which is natural to the skin of any one species of animals. 
Hence in Part II. of this volume it has been found most 



LIGHT LEATHER MANUFACTURE 93 

convenient to deal with the different classes of skins in 
different sections. Jnst as the hides of ox and heifer were 
much the most numerous and important of hides, so also 
naturally are sheepskins the most prominent section of the 
raw material of the light leather trade. This is the more 
true because the skin is valued for its wool as well as for 
its pelt ; indeed, the wool is often considered of primary 
importance, and receives first consideration in fellmongering. 
Unfortunately for the light leather trade, sheepskins, 
though most numerous, do not give the best class of light 
leather, the quality being easily surpassed in strength, 
beauty and durability by the leather from goat or seal skins. 
In the wet work for the preparation of skins for tannage 
much the same general principles and methods are em- 
bodied as in the case of hides, but with appropriate modifica- 
tions. As soft leathers are chiefly wanted, a mellow liming 
is quite the usual requirement for all skins. It is also 
usual to have a long liming, for some skins (like those of 
sheep and seal) have much natural fat which needs the 
saponifying influence of lime and lipolytic action of the 
enzymes of the lime liquors ; whilst other skins (like those 
of goat and calf) are very close textured and need the 
plumping action of the lime and a certain solution of inter- 
fibrillar substance. In consequence of the long mellow 
liming, sulphides are not usually necessary, and indeed 
sodium sulphide is not usually desirable, on account of its 
tendency to make the grain harsh. It is used, however, 
for unwoolling sheepskins, in such a manner that the grain 
is not touched. Similarly caustic soda is seldom required, 
and the yield of pelt by weight is usually a small considera- 
tion. Systems of liming show some variety. The one-pit 
system is very common, and is less objectionable for a long 
mellow liming, but rounds of several pits are also used, 
and in some cases even more than one round. This is 
obviously conducive to regularity of treatment, and as the 
work involved in shifting the goods is much less laborious 
than in the case of heavy ox hides, it would seem a prefer- 
able alternative. The depilation of sheepskins involves 



94 ANIMAL PROTEINS 

very special methods of treatment (sweating and painting) 
on account of the importance and value of the wool, the 
quality and value of which would be impaired by putting 
the skins through ordinary lime liquors. The pelts, how- 
ever, are limed after unwoolling. 

In deliming light leathers the process of puering is widely 
used (see p. 25). This consists in immersing the skins after 
depilation in a warm fermenting infusion of dog-dung. In 
principle this disgusting process presents a close analogy with 
bating, and indeed the two terms are both used somewhat 
loosely, but there are nevertheless several points in which the 
two processes are radically different. The dog-dung puer is 
a process carried out at a higher temperature than the fowl- 
dung bate ; it is also a much quicker process, and the 
infusion employed is generally more concentrated. Whilst 
the fowl-dung bate is always slightly alkaline to phenol 
phthalein the dog-dung puer is always acid to this indicator, 
and the course of the puering may be conveniently followed 
by testing the pelts with it. The mechanism of the two 
processes is also probably somewhat different. The mechan- 
ism of the dog-dung puer has been largely made clear by the 
researches of Wood and others, and been found due partly 
to a deliming action by the amine salts of weak organic 
acids and partly to the action of enzymes from a bacillus 
of the coli class, which received the name of B. erodiens, 
and which effects a solvent action on the interfibrillar 
substance. As we have noted (Part I., Section II., p. 24), 
the fowl-dung bate involves two fermentations, in each of 
which (aerobic and anaerobic) several species of bacteria 
are probably active. Wood found the bacteria of the bate 
to be chiefly cocci, and ascribed part of the difference in 
mechanism by the nature of the media, which in the bate 
includes also the urinary products. In the dog-dung puer, 
also, a lipolytic action is probably an essential part of the 
total effect. The puer gives a much more complete de- 
liming and a much softer and more relaxed pelt than the 
bate, it is therefore particularly suited to the needs of light 
leather manufacture. The puering action has been imitated 



LIGHT LEATHER MANUFACTURE 95 

fairly successfully by artificial methods. " Erodin " (Wood, 
Popp and Becker) involves the use of B. erodiens and a suitable 
culture medium including organic deliming salts : ' ' Oropon,' ' 
"Pancreol" and others involve the use of ammonium 
chloride and trypsin, together with some inert matter. 

I^ight-leather goods are usually drenched after puering. 
They are also often split green after the wet work. Sheep- 
skins thus yield " skivers " (the grain split), whilst the flesh 
split is often given an oil tannage (see Part IV., Section III.). 
The greasy nature of sheep and seal skins necessitates the 
processes of " degreasing." In the case of sealskins this 
is done largely before liming, but with sheepskins either 
after being struck through with tan, or after tannage is 
complete. Sheepskins are often preserved in the pelt by 
pickling with sulphuric acid and salt, which process forms a 
temporary leather. The fibres of the pelt are dried in a 
separate condition, but the adsorption is easily reversible 
and the pelts may be " depickled " by weak alkalies and 
afterwards given an ordinary vegetable tannage. 

In the vegetable tannage of skins for light leathers, the 
same theoretical considerations have force as in the heavy - 
leather section, but the former has its own rather special 
requirements and aims. Generally speaking, a softer and 
more flexible leather is required, but these qualities must 
not be imparted by stuffing with grease as in the currying 
of dressing leather, because a bright and grease-free result 
is usually required. Hence it is important that a sweet 
mellow tannage be given. The durability of the leather is 
also a primary consideration for goods intended for book- 
binding, upholstery, etc., and the tannage must be arranged 
to impart this quality and avoid anything tending to cause 
the perishing of the fibre. Thus oak bark is a popular tanning 
material, and sulphuric acid very definitely avoided. The 
tannage must be fast, and take the dyestuffs well, and for 
the production of light shades of colour in dyeing must be 
a light-coloured tannage. All these qualities are imparted 
by sumach, which also fits in excellently with the other 
general requirements, such as softness, brightness and 



96 ANIMAL PROTEINS 

durability. Hence sumach is the principal light-leather 
tanning material, but the tendency is to employ other 
materials — oak bark, myrabs, and chestnut extract — to do 
much of the intermediate tanning, so that the expensive . 
and useful sumach may be used for setting the colour and 
grain at the commencement, and for brightening, bleaching 
and mordanting the leather at the end of the tanning process. 
Weight is generally no consideration, but area is often a 
definite aim, partly because some goods are sold by area 
and partly because the striking out, setting out and similar 
operations improve the quality of the leather by giving 
evenness of finish. Leather well struck out, moreover, is 
less liable to go out of shape. As the grain pattern is so 
important in the finished leather, appropriate care must be 
taken during tannage. If a smooth or a fine grain finish 
is wanted, for example, the goods must not be allowed to get 
wrinkled, creased, doubled or unduly bent to and fro 
during the tanning. For such goods, suspension, careful 
handling and even the " bag tannage " may be desirable, 
whilst for coarser and larger grains paddles or drums may 
be more extensively used. 

Amongst the finishing processes dyeing holds an im- 
portant position. The nature of the process has many 
points of similarity with that of tanning. The great specific 
surface of pelt is probably more enhanced than otherwise 
during tannage, at any rate with light leathers, owing to 
the isolation of fibres, and consequently leather is as liable 
as pelt to exhibit adsorption. The dyestuffs, on the other 
hand, are substances very easily adsorbed. Some (like 
eosin and methylene blue) are crystalloids, some (like 
fuchsin and methyl violet) are semi-colloids, whilst others 
(like Congo red and night blue) are undoubted colloids 
forming sols (usually emulsoid) with water as dispersion 
medium. The crystalloids and semi-colloids may also be 
obtained in colloidal solution, sometimes being so changed 
on the mere addition of salts to the solution. In addition, 
the pelt has been mordanted with tannin. If, however, 
leather has been kept long in the rough- tanned or "crust" 



LIGHT LEATHER MANUFACTURE 97 

state, this may not be so effective, owing probably to the 
secondary changes in tanning (Part I., Section III., p. 46), 
but such leathers are usually " retanned " or prepared for 
dyeing by sumaching (which process also incidentally 
bleaches). The tannin mordant assists materially in the 
fixation of the dyes. In the case of basic dyestuffs, lakes 
also are formed, i.e. there is a mutual precipitation of 
oppositely charged colloids (+dye, — tannin). The dyeing 
of leather is thus a case of colloid reactions even more com- 
plicated than that of tanning. 

Another finishing operation typical of the light leathers 
is " graining " or " boarding." In this the skins after dyeing 
and drying are worked by aboard which is covered by cork, 
rubber, perforated tin or other material, and so grips or 
" bites " the leather. The object of " graining " is to work 
up the grain pattern by pushing or pulling a fold on the 
skin with the board. The nature of the grain varies with 
the thickness and the hardness of the skin, with the amount 
of pressure applied, with the nature of the board, with the 
direction of the boarding and with the total number of 
directions boarded. There is thus infinite scope for variety 
of finish, and hence arise bold grain, fine grain, hard 
grain, straight grain, cross grain, long grain, etc. The 
operation requires considerable skill and experience. In 
the case of skins with little natural grain (such as sheep- 
skin) embossing and printing machines impress the desired 
pattern. 

In seasoning, a dressing is applied containing essentially 
albumins and emulsified fats, e.g. egg albumin and milk. 
Colouring matters are also often added to intensify or 
modify the shade. After seasoning the goods are usually 
" glazed " by a machine which rubs the seasoned grain with 
considerable pressure, by a glass or hardwood tool, and so 
produces a high gloss, for which the seasoning is very largely 
a preparation. I^ight leathers are very lightly oiled with 
linseed or mineral oil. 



9 8 ANIMAL PROTEINS 



REFERENCES. 

Procter, " Principles of Leather Manufacture," pp. 220, 394. 
Bennett, "Manufacture of Leather," pp. 36-41, 55, 85-90, 92-112, 
312, 332- 

Wood, "Puering, Bating and Drenching of Skins." 
Lamb, "Leather Dyeing and Finishing." 



Section II.— GOATSKINS 

Goatskins are amongst the most valued raw material for 
the manufacture of light leather. The leather obtained 
from them is of the very finest quality in respect to dura- 
bility and adaptability to the principal purposes in view. 
The texture of the fibres in goatskin is exceedingly compact 
and very strong, whilst the grain exhibits naturally a 
characteristic pattern which renders it most suitable for a 
grained finish. Hence for purposes like upholstery, book- 
binding, slippers, it forms almost an ideal material. The 
tanning and finishing of goatskins into " morocco leather " 
may indeed be taken as a quite typical example of light 
leather manufacture. 

The skins are obtained from all quarters of the globe 
where goats exist, and the excellent quality of the leather 
produced has created a demand which is greater than the 
supply. This is due not only to the demand for morocco 
leather, but also to the popularity of the goatskin chrome 
upper leathers such as "glace kid " (see Part III., Section IV.). 
The large American trade in the latter has produced the 
saying that wherever there is a goat there is an American 
waiting for it to die ! The European supply of skins is 
somewhat limited. They are obtained from the Balkans 
and Bavaria, in which case they are small, fine-grained and 
plump skins. The Swiss goatskins are larger, and have 
also a fine grain ; they are well grown and well flayed. 
Scandinavian skins have a poor reputation, being very flat. 
The African supply is important ; Abyssinian skins are 
exceedingly compact and tough, and are very suitable for 
" bold grain " finishes. The Cape skins are particularly 
large, strong and thick, but their quality is often impaired 
by the cure, the skins being flint-dry, and, like hides so 



ioo ANIMAL PROTEINS 

cured, prone to unsoundness. Large quantities of goat- 
skins also come from the East. Many of these are imported 
in a tanned state (E.I. Goat). These skins are tanned with 
turwar bark, which contains a catechol tannin. They are 
also heavily oiled with sesame oil, and need degreasing. 
The tannage is also stripped as far as practicable, and the 
skins retanned with sumach before finishing. They make 
good morocco leathers for many purposes, but the primary 
catechol tannage renders them ineligible for finishing under 
the specifications of the Committee of the Society of Arts. 
The skins have a Persian or Indian origin. India also 
supplies a large number of raw dried goatskins which are 
small and of variable quality. These, however, are more 
extensively used for chrome uppers. 

Goatskins are imported in either a salted or a dried 
condition. The great aim of soaking is to obtain the skins 
in a thoroughly soft condition. Hence the soaking is 
prolonged, and some mechanical treatment is desirable in 
addition to various steepings in water. To be certain of 
softness it is desirable to avoid the use of alkalies in the 
soak waters, for although they cause hydration of the fibres 
by imbibition, they also have a plumping effect which is 
not wanted at this stage. Salted goatskins are first im- 
mersed in water and left until the following day. This 
dissolves the salt. They are then stretched and given a 
fresh soak liquor of water only to soften further, clean, and 
remove the rest of the salt. This second water lasts only a 
few hours, and the goods are then drummed well in running 
water. This not only cleans quickly, but has an excellent 
softening effect. They are again returned to a soak liquor, 
then softened mechanically by working them over a beam. 
This treatment must be repeated, drumming again if neces- 
sary, until the skins are perfectly relaxed and thoroughly 
softened. If the treatment be very prolonged it becomes 
advisable to use antiseptics in the soak waters after the 
first drumming. Solubilized (or emulsified) cresols of the 
" J eyes fluid " type are the most suitable antiseptics, but 
too much must not be used or the sterilization affects the 



GOATSKINS 101 

liming, in which bacterial action is needed. Flint-dry skins 
are left longer in the first soak, which should be of water 
only. They are then given a fresh soak liquor containing 
0*2 per cent, of sodium sulphide. Sometimes aro per cent, 
solution of borax is used instead ; it softens excellently, is 
antiseptic, and avoids the plumping effect, but is rather 
expensive. The goods are next drummed well, and re- 
soaked and worked as for salted skins. In either case the 
soaking takes about a week. 

The liming of goatskins presents some points of contrast 
with the methods used for other skins. These differences 
are due to the exceedingly tight and compact nature of the 
skin fibres. This compactness of texture makes it quite 
necessary to dissolve the interfibrillar substance to a greater 
extent than usual, and also to plump the fibres and split 
them into the constituent fibrils. These effects are essential 
to obtain a rapid and complete tannage and a soft leather. 
Too much bacterial action should be avoided, however, or 
the brightness and soundness of the grain may be impaired, 
which would be a fatal defect in such a leather. Hence the 
liming is long rather than mellow, and sharp limes rather 
similar to those required for sole leather are often used. 
Another result of the tight texture of goatskin is that 
depilation is not easily effected. This feature is rather 
intensified by the deepness of the hair-root. Hence it is 
usual to employ sulphides to assist the depilation. In one 
method two rounds of five pits are used. The skins are 
given about two days in each pit, so that the liming lasts 
approximately three weeks. In the first round, which 
consists of rather mellow limes, arsenic sulphide is used to 
assist depilation. Up to 6 per cent, on the weight of lime 
is added during slaking. This is a comparatively large 
amount of arsenic sulphide, and the depilation is consider- 
ably hastened ; the skins indeed are unhaired after passing 
through this round, i.e. after about 10 days' liming. In the 
next round the object is plumping, and caustic soda (or 
carbonate) is added to the lime liquors in quantities com- 
parable to those suggested for sole leather (Part I., Section V., 



102 ANIMAL PROTEINS 

pp. 55, 56). In this round the goods stay also for about 
10 days. An alternative to the above process is to hasten 
the earlier part of the liming by employing sodium sulphide 
instead of realgar. More sulphydrate may be obtained in 
solution in this way, and the unhairing may be in about 
half the time. The sulphide of soda also commences the 
plumping action which follows in the next round, but this 
alternative has the disadvantage that the skins are un- 
haired whilst the pelt is swollen with sulphide, which renders 
the grain both harsh and tender and consequently more 
liable to damage by the unhairer's knife. 

Deliming is by puering and drenching, and is often 
associated with a further mechanical working of the goods. 
The skins are inserted into a puer liquor at 85 ° F. and 
thoroughly pulled down. The caustic alkalies should be 
completely neutralized. A slight cut into a thick part at 
the butt end should develop no pink colour with phenol 
phthalein. The skins should be thoroughly relaxed, and 
the swelling so much eliminated that they are quite soft, 
weak and " fallen." The resilience and elasticity of the 
plumped skins should have quite disappeared, and the 
impressions of hand or thumb should be readily retained 
by the pelt. The grain should appear white and possess 
a soft and silky feel. In this condition they are again 
worked over the beam to soften further if possible. They 
are then rinsed and again worked over the beam. Drenching 
follows with 10 per cent, of bran on the pelt weight, the 
operation commencing at 85 ° to 95 ° F., and lasting till next 
morning. The skins are next scudded thoroughly to remove 
all dirt, but carefully so as not to damage the grain. 

In tanning, sumach and oak bark are the staple materials. 
Sumach gives a much lighter colour, and hence it is used alone 
for goods that are to be dyed the lighter shades, but oak 
bark is a " faster " tannage and more preferable for dyeing 
in those cases where blacks and very dark shades are 
wanted. For ordinary purposes a blend is usuaUy employed. 
A feature of oak bark, also, is that it tends to make a firmer 
leather, so that the proportion used must be adjusted with 



GOATSKINS 103 

this fact in mind as well as the question of colour. For 
firmer moroccos the skins may pass through a handler 
round of oak-bark liquors (io°-20°) in which a certain 
amount of sumach is added to the liquors. The sumach is 
leached and assists both in tanning and bleaching as the 
liquor works through the round. The old liquor is run to 
a paddle, and the tannage is commenced by paddling the 
drenched skins in this liquor. It is advantageous both for 
the tannage and for the efficient " spending " of the sumach 
if this liquor be slightly warmed. In the early pit liquors 
the goods are very frequently handled. There is, how- 
ever, the usual tendency of the times to save labour in this 
direction, and hence it is common to have several paddles 
with liquors of gradually increasing strength, followed by a 
shorter round of handlers in which the handling is more 
infrequent. Instead of paddles latticed drums may be 
inserted into pits containing liquors. These, however, are 
not quite so convenient. In some tanneries, especially 
where sumach only is employed, the tannage is in paddles 
throughout. A new liquor is made up with fresh sumach 
and is used repeatedly until exhausted. A three-paddle 
system sometimes obtains, in which case the operation 
closely resembles the three-pit system of liming (Part I., 
Section II., p. 19), and the skins pass through an " old " 
liquor, a " medium " liquor and a " fresh " liquor. The 
goods need not be paddled the whole day through, and 
indeed in the later stages this is undesirable. The packs 
remain several days in each liquor and take up to 14 days 
to tan. Two to three bags of sumach are needed for about 
20 dozen goatskins. This method of tanning is efficient and 
convenient for bold-grain finishes, on account of the constant 
tumbling and bending of the skins which tends to work up 
a grain. For very soft leathers and fine-grain finishes, 
however, the " bag- tannage " or "bottle tannage" is 
favoured. In this method the pelt is stitched up by machine 
to form a bag, grain outwards, leaving a " neck " in the 
hind shank. The bag is nearly filled with a fairly strong 
infusion of sumach, inflated with air and tied up at the neck. 



io 4 ANIMAL PROTEINS 

The bags are then placed into a vat of warm sumach liquor, 
in which they just float. The bags are pushed down and 
the liquor stirred up, so that the goods are in constant 
motion. After a few hours they are piled on a rack, and 
the tan liquor of the interior is caused to diffuse through 
the skins by the pressure due to the weight of the pile. 
The bags are refilled with fresh and stronger sumach liquor 
and the process is repeated. The skins are thus lightly 
but effectively tanned in about 24 hours, and the leather 
has very fine grain and soft feel. However tanned the skins 
are dried out after tanning, and sorted in the " crust " 
according to size and colour. The larger skins are preferred 
for upholstery and the smaller for fancy goods and book- 
binding. 

To illustrate the course of finishing operations, the case 
of hard-grain morocco for bookbinding may be given as 
typical. The goods are wet back with warm water and 
drummed for 1-2 hours in warm sumac to prepare for 
dyeing. They are then struck out by machine, sammed 
and shaved. Dyeing follows, with acid colours, in a drum. 
The goods are run first in a little water and the dyestuff 
added very gradually through a hollow axle. The acid 
required (preferably formic) is added later to develop the 
full shade. Warm solutions are used, and the dye bath is 
practically exhausted. The goods are next placed in cold 
water to wash off superfluous liquor and free the skins from 
acid. They are then horsed to drain, struck out and hung 
up to samm. They are seasoned with milk and water and 
piled to temper. They are " tooth rolled " in the glazing 
machine two ways : right hand shank to left fore shank and 
vice versa, and piled again. After wetting back again they 
are " wet grained " by hand with a cork board in four 
directions : belly to belly, shank to shank, and across as 
before, and finally from neck to butt. They are immediately 
hung up in a warm shed to dry, and to fix the grain. They 
are then softened by " breaking down " with a rubber board, 
top seasoned, piled to temper and dry, brushed lightly, piled 
again, brushed more heavily, and dried out. They are 



GOATSKINS 105 

finally softened by graining in three directions : shank to 
shank and across, and neck to butt. They are then brushed 
again. If these skins are wanted for upholstery they are 
shaved after d}7eing, and nailed on boards to samm. They 
are also dried out in a cooler shed or " stove," to ensure 
softness. 



REFERENCE. 
Bennett, "Manufacture of Leather," pp. 39, 55, 89, 111, 204, 344, 396. 



Section III.— SEALSKINS 

A special class of morocco leather is manufactured from 
the skins of seals. This should not be confused with the 
" sealskin " of popular parlance, which is manufactured 
from the skin of a different animal. All the fin-footed 
mammals (Pinnipedia) , except the walrus, are termed seals, 
but they are divided into two families. The Otariidce are 
known by their possession of small but distinct external 
ears : into this class fall the fur-seals whose skin is dressed 
with the fur on, for women's jackets, muffs and caps. The 
Phocidce are that family without external ears : the skins 
of many species (Phoce Greenlandica, Phoco barbata, etc.) of 
this family are unhaired and given a vegetable tannage, 
thus forming the raw material of sealskin morocco leather. 
It is with the latter that this section will deal. 

As the seal is a marine animal and is partial to the 
colder seas, its skin is very oily. The skins are imported 
in a salted condition from both the Arctic and Antarctic 
regions, North Europe, North America and Newfoundland 
supply many skins, and the southern material is supplied 
chiefly through the Cape. Sealskin shares with goatskin 
the properties of compact texture, strength of fibre, and 
great durability, all of which fit it for the manufacture of 
moroccos for upholstery, bookbinding, etc. It is, however, 
readily distinguishable from goatskin by its characteristic 
grain pattern. 

In soaking sealskins the object is not only to soften 
thoroughly, but also to effect the recovery of as much seal 
oil as possible before the liming commences. This is 
desired because the oil is in itself a valuable bye-product, 
and because its removal is essential to a satisfactory liming 
and tannage. The removal of the oil is materially assisted 



SEALSKINS 107 

by raising its temperature, so that the soaking of sealskins 
is often done with warm water (85°-88° F.), after which 
treatment they are laid over the beam and scraped with a 
blunt knife on both flesh and grain. The oil flows away 
into a special receptacle. This treatment is repeated until 
the bulk of loose oil is removed. The process is known as 
"blubbering" or "brushing over." After some soaking 
the skins are drummed to ensure softness. The skins are 
then fleshed. More oil may be obtained from the fleshings. 

By fleshing before liming a more regular action of the 
lime is obtained. This is necessary to " kill " the grease 
still remaining in the skin. A long and mellow liming is 
given for the same reason. Fully three weeks are given, 
and old limes are much preferred, partly to obtain the 
maximum lipolytic action and partly to avoid the intense 
ribbing of the pelt which new limes so easily impart to the 
older animals. These ribs are very difficult to eliminate in 
the subsequent work. Some factories find it necessary to 
finish up in new limes, however, in order to plump and split 
the compact fibre bundles into their component fibrils. 
The plumped pelt is also easier to split green. No sulphides 
are usually employed. Sweating (see Section IV., p. 113) is 
sometimes used for depilation, and in this case the ribbing 
of the pelt does not take place. 

The puering is unusually thorough with sealskins. This 
is to obtain the maximum softness and take full advantage 
of the lipolytic action. The puer liquor is fully 95 F., and 
the skins are paddled for about three hours, or until fully 
pulled down and completely delimed. Scudding follows, 
now usually by machine. The skins are then well drenched. 
The action is intensified by the use of peameal in addition 
to the bran. About 10 per cent, of the mixture on the weight 
of pelt is used. It is customary, however, to drench at a 
lower temperature (68°-yo°) than in the case of goatskins 
(Section II., p. 102), but the goods are left in the drench 
overnight only, as is usual in drenching. It is quite possible 
that drenches worked differently may have also a somewhat 
different fermentation and be due to other organisms than 



108 ANIMAL PROTEINS 

the symbiotic bacteria discovered by Wood. It is equally 
possible that the acids produced are also different, in relative 
proportion, if not in nature, and that consequently there is 
a real difference in the practical effect. In the Author's 
opinion, the great probability is that in the drench are several 
fermentations, and that if the action be reduced by lowering 
the temperature, but intensified by adding peameal to the 
bran, some of these fermentations are encouraged at the 
expense of others. 

The tannage of sealskins depends upon the size of the 
skins, the purpose for which they are intended, and whether 
they have been split or not in the limed state. The largest 
and coarsest skins intended for boot uppers, and those 
which have been heavily scratched on the grain and are 
only suitable for enamels, are given a tannage which may 
last about 5 weeks. The liquors are made from oak bark 
and mimosa bark, and are made up to 35 ° with gambier and 
possibly myrabolans extract. For fancy work also heavy 
skins are used, but a softer tannage is needed. If for blacks 
the tannage is with gambier and chestnut extract. Two 
sets of handlers are given (io°-i5° and i5°-20°), using 
only gambier in the green sets. They are well sumached 
after tanning to bleach and to mordant. If for colours, 
only sumach and oak bark are employed. The skins are 
first paddled for 3-4 days in sumach liquors, in which they 
are coloured through. The liquors may be warmed ; this 
quickens the tannage and also leaches the sumach. The 
skins are then split, and the grains pass through a handler 
set with liquors made from oak bark (8°-24°). The skins 
are in this set for 3 weeks, in the first half of which they 
are very frequently handled. They are finished off by 
paddling for 1 or 2 days in a fresh liquor containing much 
sumach, which mordants the skins and bleaches the bark 
tannage. The flesh splits are given a drum tannage in 
chestnut and quebracho extracts. If small skins are being 
tanned for bookbinding purposes, sumach only is employed, 
and usually the tannage is entirely in paddles. 

In finishing many types of grain may be obtained, in 



SEALSKINS 109 

blacks and in colours. The finishing of " black levant " may, 
however, be selected as a typical case. The skins are soaked 
back, tempered, and either split or shaved, according to 
their substance and the size of grain wanted. The thin 
skins of course give the fine grains. Mixed tannages need 
scorning and possibly sumaching The skins are then 
oiled up with linseed oil, sammed, set out and blacked. 
In this last operation the grain is brushed over with a solu- 
tion of logwood and ammonia, and afterwards with the iron 
mordant which often contains glue. They are next hung 
up for a while and then " wet grained " in four directions — 
belly to belly, shank to shank, across, and neck to butt. 
After hanging up in a hot stove to set the grain, they are 
cooled, fluffed on the flesh, and seasoned on the grain with 
a solution of milk and blood. A little black dyestuff may 
be added to the season. The season is well brushed in, the 
skins dried somewhat, and then glazed. They are then 
grained four ways again as above, dried out in the stove, 
and lightly oiled with warm linseed oil on the grain. 



REFERENCE. 

Bennett, "Manufacture of Leather," 40, 56, 90, 112, 206, 251, 312, 
346, 383- 



Section IV.— SHEEPSKINS 

The most numerous class of skins for light leathers is from 
the common sheep. These skins have particular value 
inasmuch as they include the wool as well as the pelt. This 
wool, which is actual^ the most valuable part of the sheep's 
skin, is the raw material of our woollen industries, and is 
one of the most important of animal proteids. We have, 
therefore, in this section to consider this dual value of sheep- 
skins, the proteid of the epidermis (wool), and the proteid 
of the dermis (pelt) ; one the raw material of the woollen 
industry, the other the principal raw material of the light 
leather trade. The first problem is to separate the two 
proteids. With other skins and hides the ordinary liming 
processes were sufficient and appropriate, but in the case 
of sheepskins the method is unsuitable, because the exposure 
of the wool to the action of caustic lime and possibly other 
alkalies would seriously impair its quality and reduce its 
commercial value. Hence this separation of wool from 
pelt is usually quite a separate business, viz. that of the 
" f ellmonger, " whose occupation it is to collect the sheep- 
skins from butchers and farmers, to separate the two im- 
portant constituent proteids, and to hand the wool in one 
direction to the " wool stapler," who sorts it according to 
quality, and to hand the pelt in another direction to the 
light leather tanner, who tans and finishes the pelt to fit 
it for light upper work, fancy goods, etc. 

In the first instance, therefore, we have to consider the 
work of the fellmonger, the separation of wool and pelt. 
In this work the wool receives first consideration, and the 
raw material of the fellmonger is usually classified accord- 
ingly into "long wools," " short wools," and "mountain 
breeds." The skins vary very largely in quality of wool 
and in quality of pelt, being influenced very strongly by 



SHEEPSKINS 



in 



the conditions under which the sheep lived, and by the 
precise breed of animal from which the skin has been taken. 
As in the case of hides (Part L, Section I., p. 8), animals 
exposed to extremes of weather develop the best pelts, 
whereas those sheep which have been carefully bred and 
reared for the sake of their wool yield a thin and poor 
class of pelt. In Britain, and more especially in England, 
are reared the finest and most valuable sheep. This is 
evident from the prices paid for them by foreigners and 
colonial breeders when seeking new blood for their flocks 
and fresh stock for their lands. As much as iooo guineas 
have been paid by an Argentine firm for a single Lincoln 
ram. 

Long wools are obtained from some of the best and most 
extensively bred animals. The " Cotswolds " are the largest, 
and probably the original breed of England are still found 
on the Cotswold Hills. They have long wool, white fleeces, 
white faces, and white legs, and have no horns. The wool 
is fine, but the pelts are particularly greasy, especially along 
the back. A later breed originating in the Midlands was 
called the " Leicester " long wool. This breed gives a great 
cut of wool and much coarse mutton. It is very extensively 
distributed in the North of England and has been much 
crossed, so that many sub-breeds are now well known, e.g. 
the " Border Leicester " — the general utility sheep of 
Scotland— and the "Yorkshire Leicester" or "Mashams," 
much bred in Wensleydale. " Lincolns " are another long 
wool found only on the Lincolnshire Wolds. They also 
have white faces and shanks and yield a large pelt with fine 
grain. They give a big crop of wool. " Devons " are a 
smaller breed common in Somerset, Devon and Cornwall. 
They yield a fairly long wool of great strength, but not quite 
white. Romney Marsh sheep (" Kents ") are also long 
wools. They have white legs, white faces, a tuft of wool 
on the head, and no horns. The pelt is large and good. 
" Roscommons " are an Irish cross breed with much Leicester 
blood. They yield a long wool and a spready pelt. 

Short wools are typified by the " Down " sheep. These 



ii2 ANIMAL PROTEINS 

sheep are extensively bred on the chalk lands which comprise 
a very large percentage of the southern counties of England. 
The " South Downs " are the best and most important, the 
breed being the general utility sheep of England. They 
are small but well-shaped animals with grey faces, no horns 
and fine close wool. The pelt is only fair, but the mutton is 
excellent and provides the meat sold in our best shops. 
This breed has largely stocked New Zealand. The " South 
Down is a somewhat delicate animal, and has therefore 
been largely crossed with Cotswolds and other breeds. 
Many well-known cross-breeds are found in the eastern and 
southern counties. The " Suffolks," for example, are found 
in the eastern counties. They have black heads, faces and 
legs. " Oxfords " and " Hampshires " are similar, but 
larger. " Shropshires " are another hardy cross-breed, 
which yield a heavier fleece. All the cross-breeds are larger 
than the South Down and yield bigger pelts. 

Mountain breeds yield wool of varying quality but give 
the best pelts. The " Cheviots " — much favoured by the 
Scotch farmers — have a wool of medium length but with 
much hair in it. They have white faces and legs and no 
horns, and yield excellent pelts. The "Black-faced Mountain 
Sheep " have longer wool but coarse, and yield good pelts. 
They are kept in the hilly parts of North England and in the 
Scottish Highlands. "Lonks" yield a large and good pelt, 
but very coarse wool. The mutton is good. They are a very 
large breed with much curved horns and black faces. There 
are also some small breeds, " soft wools," " Shetlands," 
and " Welsh Mountain Sheep." The wool of the last two 
is poor, but the Welsh pelts are valued for their fine grain. 
There are large numbers of sheepskins also imported, from 
South and Central America, and from Australia, New 
Zealand and the Cape, The colonies, however, have often 
done their own fellmongering, and we have imported pickled 
pelts. They now tan the skins also, and many tanned sheep- 
skins are now imported. There are also many Indian skins 
imported after tannage with turwash bark (cp. E.I. Goat, 
Section II., p. ioo). 



SHEEPSKINS 113 

The depilation is brought about by " sweating " (or 
" staling ") and by " painting." The immediate object of 
both these types of method is to avoid using anything which 
will affect the wool. The sweating process is the most 
ancient method of unhairing and is used in America for 
hides as well as sheepskins. It consists of a more or less 
regulated putrefaction. The loosening of hair or wool has 
long been accepted as evidence that putrefaction had com- 
menced in a hide or skin, and it is the aim of the sweating 
process to stop the action at that stage, before any damage 
has been done to the pelt. This aim is achieved rather 
imperfectly by suspending the goods in closed chambers 
and regulating the temperature and humidity by means 
of steam and water. Such chambers are known as " sweat 
pits " or" tainting stoves." In the case of sheepskins the 
" warm-sweat " system is generally used, and the operation 
is carried out at 75°-8o° F. A satisfactory yield of wool is 
obtained in good condition, but the pelt is very liable to 
suffer bacterial damage and show " weak grain." The skins 
are first cleaned by a few " soaks " in clean fresh water, 
with intermediate help from a " burring machine " which 
presents a rapidly revolving set of spiral blades to the wool, 
and in the presence of a good stream of water quickly 
removes all dirt from the wool. The skins then enter the 
tainting stove, and the operation is commenced by a slight 
injection of live steam. In summer, about a week is 
sufficient to loosen the hair, but in winter up to two weeks 
may be necessary. Little control of the process is possible, 
and all that can be done is to watch the goods carefully 
near the end of the operation. In one variety of this 
method of unwoolling the skins are painted on the flesh 
side with a creamy mixture of lime and water and piled for 
a day or two until the pelt is distinctly plumped. They are 
then washed with fresh water to remove the excess of lime, 
drained, and then enter the tainting stove. By this method 
the pelts are obtained in better condition and are less liable 
to damage by local excess of putrefaction. In unwoolling 
the skins are placed over a beam and the true wool is pulled 

E. 8 



ii4 ANIMAL PROTEINS 

out by hand. The wool is graded as it is pulled and different 
qualities kept separate : ewe wool, lamb wool, hog wool, etc. 
The hair is next removed from face and shanks by means 
of a blunt " rubbing knife," and the pelt then immersed in 
water. 

In the other method of depilation, by painting, advantage 
is taken of the loose texture of the sheepskin fibre and of 
the fact that the wool root is nearly halfway through the 
skin. The flesh side of the clean skin is painted with a 
creamy mixture of lime in a strong solution of sodium 
sulphide (i4°-24° Beaume). Care is taken to keep the 
depilatant off the wool. The skins are folded flesh to flesh 
and left for a few hours or until next day before unwoolling, 
according to the strength of the sulphide solution. The 
depilatory action is entirely chemical, being due to the 
solvent action of the sulphide on the hair root. The lime 
is sometimes omitted. After pulling, the skins are opened 
up and washed in fresh water. 

The various classes of wool are sold to the wool-stapler 
and so to the woollen industry. As this is a mechanical 
rather than chemical industry, its discussion is beyond the 
scope of this volume. However unwoolled, the pelt still 
needs further treatment by the fellmonger. It needs 
liming and unhairing. This is done in the ordinary way in 
pits of milk of lime, through which the goods pass from 
old to new limes in the course of about a week. This 
plumps the fibres, separates the fibrils and kills the grease. 
Paddles are used also to save handling. Shearlings are 
sometimes limed 9-14 days and unwoolled without sweating 
or painting. After liming the skins are unhaired and fleshed, 
and placed in clean strong limes until sold to the tanner. 

Sheepskin pelts are sometimes preserved b}' pickling. 
This consists in placing them first in a solution of sulphuric 
acid (about | per cent.) together with some common salt. 
The pelts swell up and imbibe the acid solution. They 
are then placed in saturated brine, which causes a very 
complete repression of the swelling, the pelts being apparently 
leathered. In this condition or partly dried out they may 



SHEEPSKINS 115 

be kept for years. The forces at work in this phenomenon 
are somewhat complex (see Part V., Section L, p. 200). 
The skins may be depickled by paddling in a 10 per cent, 
salt solution to which weak alkalies such as borax, whitening, 
carbonate and bicarbonate of soda, etc., have been added. 

The leather manufacturer classifies sheepskins according 
to the size of the pelts. The large skins are tanned for light 
upper leathers and similar work. These are called " basils." 
Many large skins are also split green into " skivers," which 
after vegetable tannage are finished for fancy goods, book- 
binding, etc. The fleshes are often oil- tanned for chamois 
leather (Part IV., Section III., p. 181). Medium-sized skins 
such as are obtained from the Down sheep are tanned for 
" roans," and finished as a kind of morocco leather. Small 
skins are mostly " tawed " (Part IV., Section I., p. 174) 
for glove leathers, but some are made into roller leather by 
vegetable tannage. 

Basils, which represent the heaviest sheepskin work, 
are tanned and finished in the following manner. The 
limed pelts are first bated lightly at about 8o° F. for two 
days, scudded and drenched. They are sometimes puered, 
but more often merely delimed with organic acids. In this 
last case they are first paddled in warm water to remove 
excess of lime, and a mixture of organic acids is very slowly 
added at definite intervals. When nearly free from caustic 
alkali the skins are removed and drenched overnight. 
There are two types of tannage. The West of England 
tannage is similar to those noted for sealskins when oak 
bark and sumach are employed (Section III., p. 108). There 
is also the tendency to paddle more and handle less, and to 
use the stronger tanning materials such as n^rabs, gambier 
and other extracts. After about 12 hours' tannage in 
paddles they are coloured through, and are then degreased 
by hydraulic pressure. The skins are piled in the press 
with layers of sawdust or bran between them, and the 
pressure applied very slowly. Much grease runs out, for 
the natural sheepskin contains up to 15 per cent, of oil 
and fat. Degreasing may be postponed till tannage is 



n6 ANIMAL PROTEINS. 

complete, and the grease can then be extracted by solvents 
(benzine, acetone, etc.). Degreasing after part tannage is 
usually considered preerable, and the skins may be tanned 
out in pit or paddle in about a week. The Scotch tannage 
is with larch bark from Pinus larix, which contains up to 
13 per cent, of a rather mellow catechol tan. This material 
has also some sugars and yields sour and plumping liquors. 
The basils are paddled in weak liquors (8°-ii°) for about 
2 days, and when struck through are degreased by hydraulic 
pressure. They are then soaked back and tanned out in 
stronger liquors (ii°-20°), which takes up to one week. 
They are then dried out and sorted in the crust. The 
finishing depends of course upon the purpose in view. If 
for linings they are soaked, shaved, sumached, struck out 
well, nailed on boards and dried right out. They are next 
stained with a solution of starch, milk and red dyestuff. 
After drying they are glazed by machine and softened with 
a hand board. For fancy slippers the crust skins are starched 
and stained directly, then " staked " (see Part III., Section 
II., p. 155), fluffed, seasoned and glazed. If intended for 
leggings and gaiters a flesh finish is given. The skins are 
soaked, stretched, shaved and sumached. They are then 
rinsed, drained, sammed and stained. A brown stain 
mixed with linseed jelly is usual. This is spread evenly over 
the flesh and glassed in. The skins are dried out, restained 
if necessary, and staked to raise a nap. Basils for gaiters 
are dyed in paddle and fluffed over the emery wheel. 

Skivers are split in the limed state and sometimes 
immediately degreased. They are next puered at 85 F. 
for about 3 hours in a paddle, and scudded. They are 
drenched at a low temperature (68°-yo° F.), but often 
2 or 3 days. They are again scudded and then rinsed and 
sent to tan. The skivers are tanned in a few days by 
sumach liquors working the goods up from mellow to fresh 
as usual. The liquors are warmed. Care must be taken 
that the goods do not tear. A great variety of finish is 
possible, but the " paste grain skiver " for fancy goods 
and the plain finish for hat leathers are sufficiently typical. 



SHEEPSKINS uy 

For paste grains they are soaked and " cleared " for dyeing 
by immersion in very weak sulphuric acid, excess of which 
is carefully washed out with water. Paddle-dyeing follows, 
and is preferred to drum dyeing as the skins are so liable 
to tear. After being struck out they are " pasted," by 
spreading on to the flesh a glue jelly, using first the hand, 
then a stiff brush and finally a cloth. The goods are then 
dried out. They are then seasoned, partly dried and 
printed cross-grain. They are next grained two ways 
lightly ; shank to shank, and across, lightly tooth-rolled 
and glazed. They are regrained two ways as before, dried 
out, and finally softened with a graining board. They are 
sometimes sized on the grain to fix the pattern and give a 
gloss. For hat leathers the skins are first soaked, sumached 
and struck out. If for white or cream finishes they are now 
lead-bleached. This consists of pigment dyeing with lead 
sulphate. They are immersed alternately in lead acetate 
and in sulphuric acid solutions until precipitation is sufficient. 
They are then dyed to shade. If for browns it is common to 
mordant with titanium and use basic dyestuffs, paddling 
afterwards in sumach to fix the dye. After dyeing the 
goods are struck out again, starched, and dried out on 
boards. They are again starched and rolled to give the 
plain finish. 

Roans are not split. They are degreased, puered, 
scudded and drenched overnight at 95 ° F. They are tanned 
with sumach usually in pits, and take rather longer than 
usual to tan. They are finished in much the same style as 
goatskins for morocco leather, but as the sheepskin has 
little natural grain it needs embossing or printing according 
to the type required. If for " hard grains," the skins are 
soaked, sumached, seasoned, dried, glazed and damped 
back for printing. This is done by the " hard grain " roller, 
and the goods are dried out to fix the pattern. They are 
damped back, sammed, and grained in four directions 
(cp. Section II., p. 104), dried out and boarded to soften. If 
for straight grains they are printed with a straight-grain 
roller, or grained neck to butt. After tooth rolling they are 



n8 ANIMAL PROTEINS 

boarded, dried and glazed. They are softened down and 
" aired off " in a cool store. 

Roller leather is a special class of sheepskin leather 
which is used to cover the rollers used in cotton spinning. 
The essential requirements are that a smooth plain finish 
should be given, and the leather must not stretch or be 
greasy. For this purpose small sheepskins with a fine 
small grain are chosen, such as those obtained from the 
Welsh mountain sheep. The pelts are machine fleshed, 
short haired and often puered, but the deliming is also 
brought about by organic acids also. The pelts are drenched 
in pits fitted with paddles, which are used to stir up the 
infusion occasionally. A thorough scudding is given. For 
the smooth-grain finish it is necessary to tan in weak liquors, 
and to give plenty of time so as to ensure complete pene- 
tration. An oak-bark tannage is preferred, but a little 
extract is usual to assist. The goods are coloured through 
in paddle, like basils, and are then degreased by hydraulic 
pressure. This should be as complete as possible, and 
a little heat is used to assist the escape of grease. The 
pressed skins, moreover, must be quite freed from creases, 
and this is attained first by paddling in warm water to 
remove sawdust, and then by drumming in fairly hot water, 
in which they are left overnight. The skins are tanned 
out in suspenders, taking about 3 weeks. The crust skins 
need careful sorting, and are soaked and hand shaved. 
They are sumached in drum, rinsed, struck out, sammed 
and set. The striking and setting should be thorough, in 
order to get rid of stretch. They are next " filled " by 
coating with linseed jelly or similar material, and dried 
out on boards in a thoroughly stretched condition. They 
are then trimmed, seasoned and rolled with a steel roller. 
They are then staked or perched, fluffed, re-seasoned, dried 
and glazed. They are carefully short-haired, glazed again 
and finally ironed. 

E. I. sheepskins are imported in a tanned condition. 
These are soaked back and the turwar bark tannage 
" stripped " as far as possible by drumming with soda for 



SHEEPSKINS 119 

20-30 minutes at 95 F. ; after washing they are " soured " 
in weak (| per cent.) sulphuric acid solution, and retanned 
with sumach paste for an hour, drumming at ioo° F. They 
may then be finished for basils, moroccos or roller leather as 
described above, but are often finished as imitation glace 
kid. In this case they are drum dyed, lightly fat- 
liquored (see Part III., Section IV., p. 163), struck out and 
dried. They are staked by machine, fluffed, seasoned and 
glazed. They may be re-staked and reglazed if desired. 



REFERENCES. 

A. Seymour Jones, "The Sheep and its Skin." 

Bennett, "Manufacture of Leather," pp. 36, 85, 107, 208, 349-354* 3 8 5- 



Section V.— CALFSKINS 

Calfskins are the raw material for many classes of leather. 
The term itself is rather broad. A calfskin may be obtained 
from a very y oung animal and weigh only a very few pounds, 
or it may be anything just short of a kip. Goat, seal, and 
sheep skins are obtained from adult animals, but calfskins 
from the young of a large animal. Thus there are many 
grades of quality, according to age, and the material must 
be chosen with regard to the purpose in view. Some of 
these purposes have already been discussed. Heavy calf 
is treated much like kip as a curried leather for upper work. 
Even lighter skins are given the " waxed calf " and " satin 
calf " finishes, and make upper leather of excellent quality. 
To produce such leathers the treatment is much the same as 
described in Part I., Section VIII., p. 76. Calfskins were also 
used for very light upper work, in which they were not so 
heavily greased in finishing, but rather dyed and finished 
as a light leather. In this direction, however, the vegetable 
tannage has been almost completely superseded by the 
mineral tannages, first by " calf kid," an alumed leather 
(Part IV., Section I., pp. 174-177), and afterwards by the 
now popular chrome tannage of " box calf," " willow calf," 
" glace calf," " dull calf," etc. (Part III., Section III., p. 156). 
In this section, therefore, we have only to consider calf- 
skins as used to make a vegetable-tanned light leather, such 
as may be employed in bookbinding and in the manufacture 
of fancy goods. For these purposes the skins receive a 
mellow liming of 2^-3 weeks. No sulphide need be employed, 
as the goods are soon fit to unhair. In such a mellow liming 
it is important that the bacterial activity is not too prominent, 
and hence it becomes advantageous to work the liming 
systematically in the form of a round of pits. To avoid 



CALFSKINS 121 

over-plumping in the newest limes some old liquor is used 
in making up a new pit, and its bacterial activity is reduced 
by adding it to the new caustic lime whilst slaking. Thus 
for a pack of 200-250 skins, 14-16 stone of lime may be 
slaked with about 30 gallons of old lime, and the pit filled 
up with water. If it be necessary to shorten the process 
and to use sulphide, this should be added only to the tail 
liquors of the round, and with it should be added, if possible, 
some calcium chloride to reduce the harshness of the soda. 
The skins should be puered thoroughly to obtain the neces- 
sary softness, bate-shaved if desirable, and drenched with 
8 per cent, of bran overnight. 

In tanning for fancy work and for dark colours, the goods 
are coloured off and evenly struck through with sumach 
liquors, and then tanned further with liquors made from 
oak bark, myrabolans or chestnut extract. The methods 
are very closely similar to those used for goatskins and 
sealskins (Part II., Sections II. and III.), and need not be 
described in further detail. The tannage is finished off in 
sumach. For bookbinding work, however, a pure sumach 
tannage is given, using liquor slightly warm (70 F.) . Paddle 
tannages are common, but for bookbinding the bag or bottle 
tannage is often preferred. The skins are sewn together in 
pairs, grain outwards, and nearly filled with warm sumach 
infusion, just as described for goatskins. They are then 
handled in old sumach liquors for about 3 days, and piled 
to drain and press. At this stage the bag is cut open, the 
goods worked on the flesh, and the tannage is completed 
with separated skins in newer sumach liquors, handling at 
least once a day for 4-5 days, as necessary. 

In finishing there is the usual variety, but a plain 
ungrained finish is most typical, as the smooth and fine grain 
of the young animal lends itself to this type of finish better 
than the skins of goat and seal, and gives a better quality 
leather than those from the sheep. The crust skins are wet 
back with water at about no° F., and, if necessary, sammed 
and shaved. Sumaching follows, the operation being 
carried out in a drum for 1-2 hours. The skins are then 



122 ANIMAL PROTEINS 

well struck out. Striking and setting should always be 
thorough for a plain finish, and this case forms no exception. 
Dyeing follows next, the paddle being often preferred to the 
drum, which is liable to work up a grain. The dyed skins 
are placed in cold water for a while and again well struck 
out. They are often nailed on boards to samm, and are then 
set out, lightly oiled with linseed oil and dried out in a cool 
shed. Seasoning follows, with milk and water only. The 
operation may be done with either brush or sponge, after 
which the goods are piled grain to grain and flesh to flesh to 
regulate. They may be next perched to soften and fluffed 
if desired. After top seasoning with milk, water and 
albumin the skins are hung up for a while, piled to regulate 
and brushed, first lightly and then more vigorously. They 
may be then oiled very lightly and dried out in a cool stove 
to ensure a soft leather. 



REFERENCE. 
Bennett, " Manufacture of Leather," pp. 55, 84, 105, 201, 207, 303. 



Section VI.— JAPANNED AND ENAMELLED 
LEATHERS 

The leathers which receive a japanned or enamelled finish 
are usually vegetable tannages, and so may be discussed at 
this stage. They are popularly known as " patent " 
leather, but for no obvious reason. The chief object is to 
obtain a leather with an exceedingly bright and permanent 
gloss or polish, and this is attained by coating the leather 
several times with suitable varnishes. The great difficulties 
are to prevent the varnish cracking when the leather is 
bent or in use, and to prevent it peeling off from the leather. 
Almost all classes of vegetable tannage are japanned and 
enamelled. Hides are split and enamelled for carriage, 
motor car and upholstery leathers, and enamelled calf, seal 
and sheep skins are used for boot uppers, toe caps, dress 
shoes, slippers, ladies' and children's belts, hat leathers, and 
so on. Broadly speaking, a japanned leather is a smooth 
finish and is usually black, whilst an enamelled leather is a 
grain finish with a grain pattern worked up, and more often 
in colours. Hence japanned leathers are often made from 
flesh splits or leathers with a damaged grain. It is in any case 
advantageous to buff the grain lightly, for this permits the 
varnishes to sink rather deeper and get a firmer grip, and 
avoids the too sudden transition from phase to phase which 
is one cause of stripping or peeling. Many flesh splits, 
however, are printed or embossed to give an artificial grain 
and are then enamelled, which tends to fix the embossed 
pattern. 

Almost any method of preparing dressing hides for upper 
or bag work will yield a suitable leather for enamelling and 



124 ANIMAL PROTEINS 

japanning (see Part I., Section VIII., p. 76 ; and Section IX., 
p. 86). If anything the liming should be somewhat longer 
and mellower in order to eliminate grease, as the natural 
grease of the hide causes the stripping of some varnishes. 
In finishing it is important to obtain even substance, or the 
varnish is liable to crack. Hides are soaked and sammed in, 
and often split. ' Sometimes they are split twice, giving 
grain, middle and flesh, the two former being enamelled 
and the last japanned. Other goods are shaved very 
smooth. The goods should be next thoroughly scoured 
and stoned to get as much " stretch " as possible removed. 
The)^ are often sumached, washed in warm water, slicked 
out again and sammed. They are then lightly buffed on 
the grain, and after oiling lightly are thoroughly set out and 
dried. Embossing or printing for enamels is done before 
the goods are quite dry. Considerable difference of opinion 
obtains as to the best oil to use in the above oiling. Linseed 
oil is widely preferred as being most likely to agree with 
varnishes made from linseed oil. Some manufacturers of 
japans do not dislike the use of mineral oil, but strongly 
object to cod oil, tallow or other stuffing greases as tending 
to cause the varnish to strip or peel. Other manufacturers, 
on the other hand, will not have leather with mineral oil 
in it, and indicate that nothing but cod oil should be used. 
In all probability these various preferences are determined 
by the nature of the varnish, which differs widely in various 
parts of the globe. 

In this country the varnishes are made largely from 
linseed oil by boiling it with " driers." This oil contains 
much triglyceride of an unsaturated relative of stearic acid. 
The double bonds are very susceptible to oxidation with the 
production of resinous bodies of unknown constitution. 
This phenomenon is known as " drying the oil," and has 
been extensively used in the manufacture of linoleums. 
The driers are either oxidizing agents or oxygen carriers, 
such as litharge, Prussian blue, raw umber, manganese 
dioxide, manganese borate, and " resinate." Prussian blue 
is most preferred for British japans, as it always materially 



JAPANNED AND ENAMELLED LEATHERS 125 

assists the attainment of the desired black colour. The 
exact details of the boiling, and the manufacture of the 
varnishes is still largely the trade secret of the master 
japanners, and differs indeed for the various stages of 
japanning. The varnish for the earlier coats is boiled 
longer, and the drying carried further, than in the case of 
the later coats. This is partly to obtain a product of such 
stiffness that it will not penetrate the leather. The driers 
and the pigments should be finely powdered and thoroughly 
mixed in. The boiling takes several days when at a low 
temperature, but if done in 24 hours the temperature may 
be up to 570 ° F. In the later coats driers are often not 
used, and the product is often mixed with copal varnish, 
pyroxylin varnish, etc., which greatly help in obtaining 
smoothness and gloss. Turpentine, petroleum spirit and 
other solvents are also used to thin the varnishes. Before 
boiling, the oil is often purified by a preliminary heating 
with nitric acid, rose spirit and other oxidizing agents, 
which precipitate impurities and thereby assist in obtaining 
a bright gloss. 

Before the application of the varnishes, the leather is 
first dried thoroughly in a stretched condition. This is 
accomplished by nailing down on boards which fit like 
"movable shelves into a " stove," a closed chamber heated 
by steam pipes. The temperature of the stove varies widely 
in different factories, from i40°-200° F., according to the 
nature of the varnishes. The first coat of warm and rather 
stiff japan is laid over the hot leather in a warm room, 
being spread over first by hand, then by a serrated slicker, 
and then again smoothed by hand. The goods are then put 
into the stove for several hours to dry. When dry the 
surface is pumiced and brushed and a second coat applied 
in a similar manner, but with increased care. This is 
repeated with finer japans until the desired result is ob- 
tained. Brushes are used to apply the later coats. Up to 
seven coats may be applied for the production of a smooth 
japan — three coats of ground japan, two coats of thinner 
japan, and two coats of finishing varnish. 



126 ANIMAL PROTEINS 

After the stoving is complete, the product is given a 
few days under ordinary atmospheric conditions to permit 
the reabsorption of moisture to the usual extent. Enamelled 
leathers are then grained to develop the pattern. 



REFERENCE. 
Bennett, "Manufacture of Leather," p. 380. 



Part III.— CHROME LEATHER 

Section I.— THE NATURE OF CHROME 
LEATHERS 

In these days the manufacture of chrome leather has 
attained a position hardly less in importance than that 
occupied by the ancient method of tanning by means of 
the vegetable tanning materials, and large quantities of 
hides and skins are now " chrome-tanned " after pre- 
paratory processes analogous to those described in con- 
nection with vegetable tannages (Part II., Section II. ; 
and Part II., Section I.). 

Chrome leathers are made by tanning pelts with the 
salts of chromium, and are typical of what are known as 
" mineral tannages," in which inorganic salts are the 
tanning agents. Tannage with alum and salt (see Part 
IV., Section I.) is one of the earliest mineral tannages, but 
is now of relatively minor importance. Chrome tanning 
was first investigated by Knapp (1858), who experimented 
with chromic chloride made " basic " by adding alkali, 
but his conclusions were unfavourable to the process. A 
patent was taken out later by Cavallin in which skins were 
to be tanned by treating with potassium dichromate and 
then with ferrous sulphate which reduced the former to 
chromic salts, being itself converted into ferric salt. The 
product, which was a combination of iron-chrOme tannage, 
did not yield a satisfactory commercial leather. Another 
patent, taken out in 1879 by Heinzerling, specified the use 
of potassium dichromate and alum. This in effect was 
a combination chrome-alumina tannage. The alum had 
its own tanning action and the dichromate was reduced 



128 ANIMAL PROTEINS 

to chromic salts by the organic matter of the skin itself 
and by the greases employed in dressing. The process,, 
however, was not a commercial success. In 1881 patents 
were obtained by Kitner, an Austrian, whose process was 
a combination chrome and fat tannage. The chrome was 
employed as " basic chromium sulphate " made by adding 
common soda to a solution of chrome alum until a salt 
corresponding to the formula Cr(OH)S0 4 was obtained. 
Such a solution is now known to be perfectly satisfactory, 
but at first it proved difficult to devise satisfactory finishing 
processes, and to supplement the chrome tannage with 
the fat tannage. 

The first undoubted commercial success in chrome 
tanning was obtained by the process of Augustas Schultz, 
whose patent was the now widely known " two-bath 
process," in which the skins are treated successively with 
a chromic acid solution and with an acidified solution of 
"hypo" (sodium thiosulphate) . The first bath was made 
up commercially of potassium dichromate and hydro- 
chloric acid, so that, strictly speaking, it contained potas- 
sium chloride also. The second bath contained, in effect, 
sulphurous acid, which reduced the chromic acid in the 
skin fibres to the tanning chrome salts. Free sulphur is 
also formed in this bath and in the skin, and contributes 
to the characteristic product obtained by this process of 
tanning. Many minor deviations from the original process 
of Schultz have been introduced, but the main features 
have been unchanged, and this method of tanning is widely 
employed at the present time for both light and heavy 
chrome leather. In 1893 tanning by basic chromic salts 
was revived and the use of the basic chloride was patented 
by Martin Dennis, who offered such a tanning solution 
for sale. The validity of the patent has always been 
doubtful on account of the previous work of Knapp and 
others, but the process itself was commercially satisfactory, 
and the many variants of this and of the basic sulphate 
tannages are now generally known as the " one-bath 
process " in contradistinction to the variants of the Schultz 



THE NATURE OF CHROME LEATHERS 129 

process, and are widely used for all classes of chrome 
leather. A one-bath process which deserves special mention 
was published in 1897 by Prof. H. R. Procter. In this 
the tanning liquor was made by reducing potassium 
dichromate in the presence of a limited amount of hydro- 
chloric or sulphuric acid by adding glucose. Although a 
basic chrome salt is the chief tanning agent thus produced, 
there is little doubt that the organic oxidation products 
play an essential part in producing the fullness and mellow- 
ness of the leather thus tanned, but their nature and mode 
of action has not yet been fully made clear though lyotrope 
influence is probable. 

More recently Balderston has suggested the suitability of 
sulphurous acid as reducing agent. A stream of sulphur 
dioxide gas is passed through a solution of sodium dichromate 
until reduction is complete. The resulting chrome liquor 
has been favourably reported upon by some chrome tanners. 
Bisulphite of soda has also often been used as the reducing 
agent. Other organic substances are also often used, instead 
of glucose, to reduce the dichromate. 

Theory of Chrome Tannage. — As to the theory of 
chrome tanning there is still considerable difference of 
opinion and much room for experiment. Some leather 
chemists regard the tannage as differing essentially from 
the vegetable tannages. Mr. J. A. Wilson has even sug- 
gested that the proteid molecule is in time partly hydro- 
lyzed with the formation of a chromic salt with the acid 
groups. The author, however, strongly favours the view 
that in chrome tanning changes take place which are 
closely analogous to those which occur in vegetable tannage, 
the differences being mainly of degree. Thus the hide gel is 
immersed into a lyophile sol — the chrome liquor — and there 
follows lyotrope influence, adsorption, gelation of the tanning 
sol, as well as diffusion into the gel, and finally also, probably, 
precipitation of the tanning sol at this interface (see pp. 41-47 
and 200-219). 

In chrome tannage the lyotrope influence is much more 
prominent than in vegetable tannage, but the effect is in the 
K. 9 



i 3 o ANIMAL PROTEINS 

same sense, viz., to reduce the imbibition of the hide gel. Thus 
the potassium sulphate in a chrome alum liquor has its 
own specific action of this kind and contributes to the 
leather formation. Unhydrolyzed chromium sulphate and 
the sodium sulphate formed in " making basic " act also 
in the same sense. 

The tanning sol is probably chromium hydrate, formed 
by the hydrolysis of chromium sulphate : it is a lyophile 
or emulsoid sol and is in consequence very strongly adsorbed 
by the hide gel. This adsorption, involving a concentration 
of lyophile sol, is the first stage in gelation, which occupies 
a relatively more prominent place in chrome than in vegetable 
tannage. Some diffusion into the gel also occurs, and both 
the gelation and diffusion of the sol are affected by lyotrope 
influence, but to a greater extent than in the vegetable 
tannage. Thus far the analogy is almost complete. 

There remains the question of the precipitation of the 
tanning colloid at the interface. This is a point which has not 
yet been thoroughly investigated, and which offers consider 
able difficulty to a clear understanding, but the matter may 
be probably summarized thus : the adsorbed chromium 
hydrate is precipitated at the interface of gel and sol to 
some extent, chiefly through the neutralization of its charge 
by the oppositely charged ions of the electrolytes present, 
but possibfy also — in the last stages of manufacture by the 
mutual precipitation of oppositely charged gel and sol. 

To illustrate the matter, the case of a basic chrome 
alum liquor will be considered. The chromium hydrate 
sol is primarily a positive sol, just like ferric and aluminium 
hydrate sols : i.e. in water they are somewhat exceptional 
in that they adsorb H* rather than OH'. To cause pre- 
cipitation therefore it is necessary to make the sol less 
positive and more negative. The positive charge of the 
sol, however, is greater than in water, because of the free 
acid formed in the hydrolysis, which results in the adsorption 
of more hydrions by the sol. Hence to ensure precipitation 
steps must be taken to reduce the adsorption of hydrions 
by the chromium hydrate sol. In practice such steps are 



THE NATURE OF CHROME LEATHERS 131 

taken, and to such an extent that there can be little doubt 
that the chrome sol is not far from its isoelectric point. 
Amongst these " steps " are (1) making the liquor " basic," 
i.e. adding alkali to neutralize much of the free acid, which 
involves a considerable reduction in the stabilizing effect 
of the hydrions ; (2) the adsorption of hydrions by the 
hide gel when first immersed in approximately neutral 
condition ; (3) the operation of the " valency rule " that 
the predominant ionic effect in discharging is due to the 
multivalent anions. In this case the divalent S0 4 " ions 
assist materially in discharging the positive charge on 
the chrome sol ; (4) the final process of neutralization in 
which still more alkali is added. The operation of the 
valency rule is the most complex of these factors, for there 
is also to be considered the stabilizing effect of the kations, 
especially of the trivalent kation Cr*" from the unhydro- 
lyzed chromium sulphate. It is quite possible also that 
in the last stages of chrome tanning there are " zones of 
non-precipitation " due to the total effect of multivalent 
ions, and it is quite conceivable that the chrome sol may 
change its sign, i.e. become a negative sol and thus give 
also a mutual precipitation with the hide-gel. This is 
particularly probable where a local excess of alkali occurs 
in neutralization. However that may be, it is probable 
that most of the tannage is accomplished by chromium 
hydrate in acid solution, and it is therefore legitimate to 
conclude that adsorption and gelation have a relatively 
greater part in chrome tannage. The operation of the 
valency rule makes it easy to understand why basic 
chlorides do not tan so well as sulphates ; the precipitating 
anion is only monovalent (CI') and chromic chloride con- 
tains no substance analogous to the potassium sulphate of 
chrome alum and hence contains a less concentration of the 
precipitating anion. Hence also the stabilizing influence 
of common salt added to a basic alum liquor, the effect being 
to replace partially the divalent S0 4 " by the monovalent 
CI'. I^yotrope influence, however, may be here at work. 
It is possible to make out a rather weak case that the 



i 3 2 ANIMAL PROTEINS 

tanning sol is not chromium hydrate at all, but a basic 
salt of chrome also in colloidal solution, and to contend 
that this salt, like most substances, forms a negative sol, 
but in practice not negative enough, hence the desirability 
of alkali, divalent anions, etc. From this point of view 
the analogy with vegetable tannage becomes more com- 
plete and the stabilizing effect of the soda salts of organic 
acids becomes easy to understand. 

It is highly probable that the electrical properties of 
the chrome sol need closer investigation on account of the 
complexity due to the prominent effect of multivalent 
ions. It is desirable to bear in mind the remarkable 
phenomenon observed by Burton (Phil. Mag., 1905, vi, 
12, 472), who added various concentrations of aluminium 
sulphate to a silver sol (negative). He observed (1) a zone 
of non-precipitation due to protection ; (2) a zone of pre- 
cipitation due to the trivalent kation ; (3) a second zone 
of non-precipitation due to protection after the sol has 
passed through the isoelectric point and become a positive 
sol ; (4) a second zone of precipitation due to the pre- 
cipitating effect of the anion on the now positive sol. It 
seems to the writer that similar phenomena may possibly 
occur in chrome tanning, for whatever the sol actually 
is, it is not far from the isoelectric point. 

A few observations on the vegetable-chrome combina- 
tion tannages will not be out of place at this stage. Wilson 
refers to the well-known practical fact that chrome leather 
can take up about as much vegetable tan as if it were 
unchromed pelt, and considers this evidence that the two 
tannages are of fundamentally different nature. " In 
mineral-tanned leathers the metal is combined with carboxyl 
groups, while in vegetable-tanned leather the tannin is 
combined with the amino groups. This strongly suggests 
the possibility that the two methods of tanning are to 
some extent independent of one another, and that a piece 
of leather tanned by one method may remain as capable 
of being tanned by the other method as though it were 
still raw pelt " (Collegium (London), 1917, 110-111). To 



THE NATURE OF CHROME LEATHERS 133 

the writer, however, it seems that the facts are evidence 
for the contrary proposition, that the tannages are funda- 
mentally of the same nature. On the adsorption theory, 
one would expect chrome leather to adsorb as much tan 
as pelt ; the readily adsorbable tan is the same, and the 
chrome leather is an adsorbent of very much the same 
order of specific surface as pelt. The adsorption theory 
would find it difficult to account for chrome leather not 
adsorbing as much tan as pelt. It is quite conceivable 
that a chrome leather could adsorb more tan than pelt, 
owing to the more complete isolation of the fibrils by the 
chrome tannage and to their being coated over by a more 
adsorbent gel. Adsorption is often deliberately increased 
by a preparatory adsorption. Thus sumach-tanned goat- 
skins are wet back from the crust and " re- tanned " in 
sumach before dyeing, to coat the fibres with a fresh and 
more adsorbent gel and so ensure the even and thorough 
adsorption of the d}^estuff. Mordanting fabrics has a 
similar object, — the adsorption of colloidogenic substances 
which give rise to an adsorbent gel on the fibre. Unless 
vegetable-tanned leather is so much loaded with tan that 
its specific surface is effectively reduced, one would simi- 
larly expect that vegetable-tanned leather would adsorb 
the chrome sol. This, of course, is exactly the case of 
semi-chrome leather. If, on the "chemical combination" 
theory, the vegetable tan combines with the amino groups 
and the chrome with carboxyl groups, it is natural to 
inquire which groups the dyestuffs combine with. As 
either tannage does not interfere with the adsorption of 
dye, are we to conclude similarly that tanning and dyeing 
are fundamentally different processes ? 

Those who favour this chemical combination theory, 
and who offer equations for the formation of vegetable 
and of chrome leather, should likewise suggest an equation 
for the formation of leather from pelt by the action of 
dyestuffs — a practical though hardly an economic process. 

The remarks made earlier in this volume (Part I., 
Section III.) as to the occurrence of what have been called 



134 ANIMAL PROTEINS 

" irreversible changes " subsequent to the mutual pre- 
cipitation of oppositely charged gel and sol, are equally 
applicable to the chrome tannages. Chrome tannage was 
once thought to embrace such irreversible changes, but 
the process can now be " reversed " with ease. The 
reversibility of the chrome tannage is an easier proposition 
than that of vegetable tannage, partly because the leather 
is comparatively much less tanned, and partly because 
the acidity or alkalinity of the stripping agent may be 
adjusted, as desired, without the oxidation trouble. In 
approaching this question from the theoretical side one 
must consider mainly whether to solate the tanning agent 
to a positive or to a negative sol. Our imperfect know- 
ledge of the electrical forces in operation in the chrome 
tannage is thus a serious drawback, but the evidence on 
the whole points to the precipitation being effected by a 
negative sol near its isoelectric point but in faintly acid 
solution. Hence, we should theoretically expect that 
reversion should take place into a negative sol in nearly 
neutral or even faintly alkaline solution. Thus, suitable 
stripping agents for chrome leather would be the alkali 
salts of organic acids (especially if multivalent). Now, 
Procter and Wilson have recently accomplished this strip- 
ping of chrome leather by the use of such salts. They 
approached the question from an empirical and practical 
point of view and found that Rochelle salt, sodium citrate, 
and sodium lactate would strip the chrome tannage with 
ease. This important and very creditable achievement 
will have great practical and commercial importance. 
Procter and Wilson have deliberately and carefully re- 
frained from offering an exact explanation of this reversible 
action, but point out that all their stripping agents are 
salts of hydroxy-acids, and strongly insist that these form 
soluble complexes with the chrome. Whilst not denying 
this in the least, the present author would point out that 
according to the views advanced in this book, the salts 
of organic acids which do not contain hydroxyl groups 
should, when combined with a monacid base, also strip 



THE NATURE OF CHROME LEATHERS 135 

the chrome tannage. This he has found to be the case. 
Thus the chrome tannage is reversible in solutions of 
ammonium or potassium oxalate and of ammonium acetate. 
With these salts the full effect of multivalent anions is not 
attained, so that somewhat strong solutions are necessary. 
A 10 per cent, solution of ammonium acetate shows some 
stripping effect after a few days, but a 40 per cent, solution 
after a few hours. Saturated ammonium oxalate is only 
a 4-2 per cent, solution, but shows a stripping effect in 
2-3 days. Potassium oxalate (33 per cent.) shows distinct 
stripping in 24 hours. Potassium acetate and sodium 
acetate show only slight action, because the solution is 
too alkaline, but strip if acetic acid be added until litmus 
is just reddened. It is noteworthy from a theoretical 
point of view that a 40 per cent, solution of ammonium 
acetate is distinctly acid, and indeed smells of acetic acid. 
There can be little doubt that such stripping actions are 
also connected with the solubility of the stripping agent 
in the gel, for the liquid must pass through the walls of 
the gel to dilute the liquid in the interior. This view fits 
in with the facts that hydroxy acids and ammonium salts 
are particularly efficient, for the tendency of chrome to 
form ammonia-complexes as well as hydroxy complexes 
is well known. From this point of view we should not 
expect a stripping action from a salt such as disodium 
phosphate, which would form an insoluble substance. 
Actually sodium phosphate does not strip, and indeed 
reduces the stripping power of ammonium acetate. Simi- 
larly, we might expect some stripping action by ammonia 
and ammonium chloride, with the formation of chrome 
ammonia complexes. This actually occurs, a pink solution 
being obtained. Sodium sulphite does not strip, possibly 
partly on account of its too great alkalinity, but is interest- 
ing theoretically to observe that sodium sulphite as well 
as Rochelle salt will strip salt stains (see Yocum's patent, 
Collegium (London), 1917, 6 ; also Procter and Wilson, 
loc. cit.). This points to the formation of a negative sol, and 
suggests many other substances for removing salt stains. 



136 ANIMAL PROTEINS 

Special Qualities of Chrome Leather. — A few words 
on the special peculiarities of the leather formed by 
chroming will not be out of place at this stage. One of 
the greatest disadvantages of the chrome tannage has 
been the absence of what is known as the " crust " or 
" rough leather " stage. In chrome tanning, the finishing 
operations have had to follow on immediately after the 
tannage. Chrome leather, after tanning, may be dried 
out like other leathers, but if thoroughly dried, or if kept 
in a dried condition for any time, it will not " wet back " 
again with water. Various suggestions have been made 
to overcome this difficulty but none yet have found much 
favour in practice. The discovery of the reversibility of 
the tannage, however, ought to solve this difficulty, and 
the author would suggest that any of the substances used 
for " dechroming " might also be suitable for " wetting 
in " chrome leather which has been well dried out. A 
piece of chrome leather, dried out well after neutralizing, 
and kept in a warm place for four years, wetted back 
easily in ammonium acetate, in the author's laboratory. 

Another peculiarity of the chrome tannage is that any 
defects in the raw material always seem more obvious 
in chrome than in vegetable leather. This often necessi- 
tates the use of a better quality hide or skin. Weak grain 
or loose grain becomes very obvious. The presence of 
short hair which both unhairing and scudding have failed 
to remove also is usually more evident. 

A more serious disadvantage of chrome leather is its 
tendency to stretch. In the case of belting leather this 
feature is an obvious nuisance, and has inevitably led 
manufacturers to use powerful stretching machines upon 
the goods before they are marketed. In chrome sole 
leather also there is a tendency to spread and throw the 
boot out of shape. 

Further disadvantages arise from the fact that the 
chrome tannage is an " empty " tannage. Compared with 
the vegetable tannage, very little of the tanning agent is 
adsorbed. Hence there is little matter of any kind between 



THE NATURE OF CHROME LEATHERS 137 

the hide fibres isolated during tannage. The inevitable 
effect of this is that the leather has not the same solidity 
and firmness, and needs filling out with other materials. 
A commercial consequence is also that it is impossible to 
obtain the same yield of leather from any given quantity 
of raw material. In trade parlance chrome tannage does not 
give good " weight." Another consequence is that (even 
when well filled with greases in finishing) chrome leather 
tends to be " woolley " on the flesh side or at cut edges. 

On the other hand, chrome tanning has very many 
advantages over the older process. The most obvious of 
these is the great saving in time. Many chrome tannages 
involve only a day or two, and none more than a week or 
two. A chrome leather factory therefore needs less capital 
on account of the quicker turnover. If, moreover, the 
market be unfavourable, a chrome tanner can stop or 
reduce his output in a very short time, whereas a vegetable 
tanner is committed to many weeks' supply of the goods 
he is manufacturing. Another notable advantage of chrome 
leather is its durability. In the finishing processes more 
grease is usually employed than in vegetable tannage, and 
this has a preservative effect upon leathers which often 
get wet. Chrome sole leather and hydraulic leathers are 
cases in point. Chrome leather will also stand changes 
of temperature and friction much better than vegetable 
tannages, The light chrome tannage results, further, in 
yielding a leather which has great tensile strength, and it 
is not surprising to find that chrome harness and chrome 
picking bands are highly thought of. The empty nature 
of the tannage necessitates the use of stuffing greases, but 
such large proportions of these may be used that chrome 
tannage becomes obviously suitable if one wishes to pro- 
duce a waterproof leather. Hence the popularity of chrome 
tannage for waterproof soling and hydraulic leathers. 

The advantages of the chrome process are very real, 
and very obviously such as will appeal to manufacturers. 
Chrome leathers have now been for some time in the fore- 
front as far as boot-uppers are concerned, especially for 



138 ANIMAL PROTEINS 

the best quality goods, in which the popular " box-calf " 
and " glace kid " are so largely employed. There seems 
little doubt that this will continue to be the case. It is 
an unfortunate fact that in this important branch of 
tanning, British manufacturers have not quite risen to 
the occasion. Their products have in the past been faced with 
very serious competition from Continental and American 
manufacturers of chrome uppers, and there can be no doubt 
that these competitors produced a better article, and pro- 
duced it more economically. The disorganization of the Con- 
tinental factories owing to the war should give British 
manufacturers a valuable opportunity of putting such busi- 
nesses on a better basis. For sole leather also the chrome 
tannage makes constant headway, and the relative proportion 
of it becomes gradually greater. A great impetus to chrome 
sole leather has been given by the war conditions of Britain. 
Owing to the submarine campaigns of Germany the tonnage 
question became all- important, and the bulky imports of 
vegetable tanning materials became a serious item. British 
tanners were therefore encouraged to make more chrome sole 
and less vegetable sole. The urgent need of leather for our 
armies also assisted in the same sense. The production of 
chrome sole progressed therefore enormously during 1917 
and 1918, and although some reaction will doubtless occur, 
there seems little doubt that chrome sole leather has taken 
a definite and permanent leap forward. Once the general 
public fully appreciate its qualities of waterproof ness and 
durability its future will be assured. 

On the whole the position and prospects of chrome 
tanning are good. The chrome tannages are making headway 
in all directions, and undoubtedly threaten the existence 
of many of the older processes of vegetable tanning. 

REFERENCES. 

Procter, " Principles of Leather Manufacture," pp. 198-220. 
Bennett, "Manufacture of Leather," pp. 210, 355. 
Bennett, J.S.L.T.C., 1917, 176. 
Stiasny, Collegium, 1908, 117. 



Section II.— GENERAL METHODS OF 
CHROME LEATHER MANUFACTURE 

It has been previously pointed out that the chrome 
tannage is an " empty " one ; the primary principle in 
the wet work of goods for chrome leather is to avoid 
anything which will make this feature more obvious. In 
the vegetable tannages relatively larger amounts of the 
tanning agents are used, and these fill the interfibrillar 
spaces ; indeed, as we have seen (Part I., Sections III., V. 
and VI.), effort is made to increase these spaces and to 
fill them to their maximum capacity, thus yielding a 
leather of which 50 per cent, is the tanning agent. In 
chrome tanning, however, the tanning agent may only 
be approximately 5 per cent, of the finished leather, so 
that any trouble taken to split the hide fibres or to dissolve 
hide substance is usually not only superfluous, but also 
calculated to enhance the " emptiness." The governing 
principle of all the preparatory processes for chrome 
tannage is therefore the conservation of hide substance, 
and this principle determines the modifications of the 
processes of soaking, liming, and deliming, which are in 
vogue. Now, in most of these processes there is usually 
some loss of hide substance, and it is the particular problem 
of chrome tanning to reduce this loss to a minimum in each 
stage. Whether the loss of hide substance be due to 
alkaline or fermentive hydrolysis, or to solation of the hide 
gel, the effect is increased by swelling, and in the wet- 
work for chrome, therefore, any variations in the degree 
of swelling are objectionable. The preparatory processes 
should be carried out with as little change as possible in 



140 ANIMAL PROTEINS 

the volume and elasticity of the pelt. Whether also the 
loss of hide be due to hydrolysis or solation, it is 
increased by time, hence short processes are (other 
things being equal) much to be preferred. Fermentive 
hydrolysis is minimized by cleanliness, alkaline hydrolysis 
by avoiding strongly alkaline liquors, and solation of 
collagen is reduced by both, and also by avoiding, as 
far as possible, the presence of calcium and ammonium 
salts. 

Soaking should be quick and clean. The use of the 
paddle or drum gives the greatest efficiency and also assists 
in procuring the softness so essential to the bulk of chrome 
leathers . 

Iyiming chrome leather satisfactorily is almost an im- 
possible ideal. Every conceivable arrangement has some 
objection to it. The time of the process may be shortened 
either by the use of sulphide or by the use of mellow or 
old limes. To shorten time by the use of sodium sulphide 
unfortunately involves the employment of more alkali 
than is desirable, with a consequent plumping effect and 
tendency to harshness. If sufficient sulphide be used to 
make the liming very short, then the grease is not " killed " 
(saponified or emulsified). - If the harshness and alkalinity 
be removed by using also an excess of calcium chloride, 
then the lyotrope influence of this substance enhances the 
solation of the hide gel. On the other hand the use of 
old lime liquors avoids the plumping effect, but increases 
considerably the bacterial activity, and the bacterial 
enzymes produce both hydrolysis and solation of the pelt. 
In practice what is generally done is to shorten time by 
both methods and so to admit both disadvantages to a 
limited extent. This is theoretically more sound than 
would appear, for in mellow limes sulphide has less plump- 
ing power but is just as strong a depilatant ; whilst, on 
the other hand, a mellow liming shortened by sulphide is 
less objectionable as there is some evidence that bacterial 
activity is relatively less in the first few days. Hence a 
mellow sulphide liming of 7-10 days is very common in 



CHROME LEATHER MANUFACTURE 141 

practice, but sometimes a 3-4 days' process with more 
sulphide is also found satisfactory. 

It would seem probable that the real solution of the 
problem would be found by a different process altogether. 
In this connection it is interesting to note that a Continental 
proposal to unhair by enzyme action only has been found 
most practicable with goods for chrome, and, in the author's 
opinion, some development on these lines, in which a 
lipolytic enzyme is used in addition to a proteolytic, might 
solve the difficulty, and give a rapid depilation which 
dispenses with liming, plumping and deliming with the 
consequent loss of valuable hide substance. 

In the usual short, mellow, sulphide liming it is clear 
that there is not much advantage in a " round " or " set " 
of pits. Hence the one-pit system is popular on account 
of the less labour involved. 

The above remarks are less applicable in the case of 
chrome sole leather. In this case weight is a great con- 
sideration and plumping is necessary. In such leather 
the chrome tannage is supplemented by the use of 
waxes, which fill up the spaces between the fibres and 
give solidity and waterproofness to the finished article. 
With this leather an ordinary sole leather liming in 
sharp liquors is not unsuitable, handling the goods from 
" mellow to fresh," but there is, on the whole, a tendency to 
shorten the process to about a week by using more sulphide. 

Processes for deliming pelt for chrome leather should 
also be chosen by our guiding principle of hide substance 
conservation. Here again short processes involving little 
change in swelling should be preferred. Now, the ordinary 
bating and puering processes give (1) neutralization of 
lime by organic acids combined with weak bases ; (2) the 
solation of some hide substance ; and (3) a " pulling 
down " effect on the swollen pelt. Now, neutralization is 
quite superfluous, as the acids of the chrome liquor (one- 
bath or two-bath) can quite well accomplish this ; the 
solvent effect is undesirable altogether ; and the " pulling 
down " effect is also unnecessary if the goods are not 



142 ANIMAL PROTEINS 

plumped up. With any method of liming, however, some 
plumping is obtained, and this creates a problem of 
practical importance. In the huge quantities of pelt 
which go for chrome upper leathers, a primary considera- 
tion is the soft, " kind," or mellow feel of the grain in the 
finished leather. This is obtained only by tanning the 
pelt when the grain at least is in a thoroughly deplumped 
and inelastic condition. It is essential to delime not only 
so that the alkaline plumping effect is completely removed, 
but also so that no acid plumping effect succeeds it. The 
practical problem is to decide whether, in any particular 
instance, dung puers and bates are necessary to obtain 
this result. Bating is clearly not very desirable, on 
account of the length of the process, during which hide 
substance would be lost unnecessarily, and also because 
there will usually be a slight alkaline swelling. Puering 
with dog-dung infusions is preferable ; it is not such a 
long process, the liquor is just acid to phenolphthalein, 
and the action is more intense, and by puering for a short 
time only the loss of hide may be confined to the grain 
and flesh only, whilst the desired inelasticity of grain-pelt 
is soon obtained. Many large firms have admittedly 
found themselves unable to dispense with puering, but 
others have succeeded in substituting for it the use of 
non-swelling deliming and lyotrope agents like ammonium 
chloride and boric acid. In all cases it is futile to delime 
or puer the grain and then allow the goods to stand until 
the centre lime has diffused outwards. The goods must 
pass into the chrome liquors when in the correct condition. 
For heavy chrome leather a surface deliming with boric 
acid is all that is necessary. Even that is superfluous 
when the goods are to be pickled before tanning. 

Types of Two-bath Chrome-Tannage. — Although the 
original process of the Schultz patent is quite a practicable 
one, many modifications have been introduced. These 
modifications have been made to suit the class of goods 
under treatment, to suit the particular mode of application 
which is available or suitable, and to effect economies of 



CHROME LEATHER MANUFACTURE 143 

chrome and other material, and of time, and also to com- 
bine with the tannage a pickling effect or a partial alum 
tannage. Other modifications arise from the precise acid, 
neutral, or alkaline condition of the pelt, being for example 
obviously necessary when pickled stock replace neutral 
pelts. The many two-bath processes which have been 
found useful have been classified previously by the author 1 
into three types : (1) The " Schultz type," in which such 
quantities of dichromate and acid are used that there is 
no excess of free acid (other than chromic), but an excess 
of unaltered dichromate ; (2) the " Acid type," in which 
the chromic acid is completely free and the liquor con- 
tains also some excess of mineral acid also ; and (3) the 
" Neutral type," in which neither of these main constituents 
is in excess, just sufficient mineral acid having been used 
to liberate all the chromic acid from the dichromate. 
Now : — 

K 2 Cr 2 7 +2HCI =2KC1 +2Cr0 3 +H 2 
204 73 

Taking the commercial hydrochloric acid as a 30 per cent, 
solution, 73 parts will be obtained in about 250 parts of 
commercial acid. Hence 294 parts dichromate need 250 
parts commercial hydrochloric acid for the above re- 
action ; 2 in other words, 5 per cent, dichromate needs 
4! per cent, commercial acid. Similarly 6 per cent, and 
4 per cent, of dichromate need 5*1 per cent, and 3*4 per 
cent, respectively of commercial acid. If therefore such 
quantities be used we have the so-called "Neutral type " 
of chroming bath. If less quantities of acid be used we 
have the "Schultz type," and if greater quantities of acid 
be used we have the " Acid type." The original Schultz 
patent used 5 per cent, dichromate and 2| per cent, hydro- 
chloric acid, and well exemplifies its type, for there is much 
undecomposed dichromate. The composition of some 

1 "Types of Two-bath Chrome Tannage," Leather, 1909, 227, 259. 

2 Commercial acids of course vary in strength, and the amount needed 
varies accordingly. 



144 



ANIMAL PROTEINS 



chroming baths in common use on a practical scale are 
given below under the heading of their type : — 



Type. 


Dichromate. 


Hydrochloric 
Acid. 


Salt. 


Aluminium 
Sulphate. 


{ 


5 


2i 


— 


— 




5 


2* 


— 


3 


Schultz ( 


5 


2£ 


5 


— 




5 


2i 


IO 


— 


\ 


6 


3 


— 


— ■ 


( 


4 


4 


— 


— 




4 


4 


5 


— 




5 


5 


5 


3 


Acid / 


5 


5 


IO 


— 


\ 


6 


6 


*5 


— 




3 


3 


15 


4 




2 


4 


IO 


• — 


\ 


4 


15 


24 


— 


/ 


5 


4l 


5 


— 




5 


4 


— 


2k 


Neutral / 


Chromic acid 










5 


— 


5 







6 


— 


8 





\ 


4 


— 


10 






All the figures are percentages of the weight of pelt. As 
K 2 Cr 2 7 has a molecular weight of 294, and Na 2 Cr 2 07,2H 2 
a molecular weight of 298, in practice they may be con- 
sidered as interchangeable, weight for weight. The sodium 
salt is cheaper and more often used. The corresponding 
amount of chromic acid, 2Cr0 3 , has an equivalent weight 
of 200, hence any weight of dichromate may in practice be 
substituted by two- thirds the weight of commercial chromic 
acid. Equivalent weights of commercial sulphuric acid 
are sometimes used in place of hydrochloric. The quantity 
depends upon the strength of the sulphuric acid used. 
Aluminium sulphate, Al 2 (S0 4 ) 3 i8H 2 (mol. wt. 666), may 
be replaced by ordinary potash alum, K 2 S0 4 ,A1 2 (S0 4 )3- 
24H2O (mol. wt. 948). In practice 7 parts of the former 
and 10 parts of the latter ma3^ be considered equivalent. 
It should be remembered that both these salts are hydro- 
lyzed in solution, and therefore increase slightly the amount 
of free acid present. Their presence decreases the amount 
of chrome taken up, and as little or no alumina is found 



CHROME LEATHER MANUFACTURE 145 

in the leather, there is usually small advantage in their 
employment. The use of salt is common but often un- 
necessary. It is considered desirable in baths of the acid 
type to prevent swelling by the excess of acid, and in baths 
made up from commercial chromic acid to replace corre- 
spondingly that normally formed from the reaction of 
dichromate and acid. It is used also in all baths which 
are intended to treat pickled goods. Iyike all electrolytes 
its presence decreases the adsorption of chromic acid. 

All these conceivable modifications will make good 
leather, and the choice of a process often depends largely 
upon market prices. On the whole the tendency is to 
prefer the neutral or acid type, on account of the greater 
ease and completeness with which the bath may be ex- 
hausted. Pickled stock may be depickled before tanning, 
by a bath of salt, mixed with borax, whitening, or basic 
alum solutions. It may also be placed direct in the chroming 
bath, but the amount of acid thus added with the goods 
must be determined and allowed for when making up the 
bath. No allowance is usually necessary, however, if the 
" pickle " consist only of alum and salt. 

The chroming operation is carried out usually in drums 
or paddles. Drums are preferable because more con- 
centrated baths may be used ; these solutions penetrate 
quicker and are easier to exhaust economically. They are 
also preferable for hides and heavy skins. Paddles are 
preferable where grain is important, and for light skins 
in which little time is needed. Small variations in the 
ratio of chrome to pelt, or in concentration of liquor, have 
little influence upon the resulting leather. 

The analytical investigation and control of chroming 
baths is usually simple. A suitable volume of liquor is 
titrated with N/10 thiosulphate after acidifying with 
hydrochloric acid and adding potassium iodide. The 
operation should be conducted in a stoppered bottle, and 
the liquor allowed to stand for 10-15 minutes after adding 
the iodide and before titrating. A little fresh starch 
infusion should be added towards the end of the reaction. 

E. 10 



146 



ANIMAL PROTEINS 



Each c.c. N/10 thiosulphate corresponds to 0*0033 gram 
Cr0 3 or 0*0049 g r ^m K 2 Cr 2 7 . The same volume of 
liquor should also be titrated with N/10 caustic soda and 
phenolphthalein. Potassium chromate is neutral to this 
indicator, i.e. chromic acid acts as a dibasic acid. Any 
excess of hydrochloric acid is also titrated. More indicator 
should be added towards the end of the titration, as it is 
often oxidized. Each c.c. N/10 soda corresponds to 0*005 
gram Cr0 3 , o*oi gram " half-bound " Cr0 3 (i.e. present 
as dichromate), 0*0147 gram K 2 Cr 2 7 , or 0*00365 gram 
HC1. If a c.c. N/10 thiosulphate and b c.c. N/10 soda 
be needed the type of chroming bath may be seen at a 
glance — 



If 


The type is 


The bath contains 


b is greater than \a 

but is less than §a 
b is greater than fa 

b equals J a 


Schultz 

Acid 

Neutral 


potassium dichromate 
and chromic acid 

chromic acid and free 
hydrochloric acid 

chromic acid only 



If 10 c.c. chrome liquor require a and b c.c. of thiosulphate 
and soda respectively — 

I. 10 c.c. of a Schultz bath contain (b— \a) Xo*oi gram 
Cr0 3 

and {(aX'0033)— [(6— |a)xo-oi]} xi'47 grams K 2 Cr 2 7 

II. 10 c.c. of an acid bath contain (axo*0O33) grams 
Cr0 3 



III. 



and {(&—§«) X 0*00365} grams HC1 

10 c.c. of a neutral bath (0X0*0033) grams-ip ~ 
or (bx 0*005) grams/ 



The second bath of the two-bath chrome tannage 
consists of a solution of sodium thiosulphate acidified with 
hydrochloric acid. The reactions in this bath are some- 
what complicated, several occurring simultaneously. 
Broadly speaking, the final result is due to (1) the reduction 
of the chromic acid to a chromic salt by the sulphurous 



CHROME LEATHER MANUFACTURE 147 

acid ; . (2) the formation of a basic chromic salt owing to 
the excess of thiosulphate ; (3) the reaction of the added 
acid and thiosulphate to give free sulphur, which is 
deposited in and on the leather. The relative intensity 
of these effects is variable, according to the conditions 
of operation, e.g. the amounts of chemicals used, their 
concentration, the nature and condition of the goods, 
the time of application, the manner of application, etc. 
In practice the most favourable conditions are usually 
discovered empirically, but, broadly speaking, the goods 
are usually added soon after the thiosulphate and acid 
are well mixed. There is some evidence that the reduction 
is in steps, intermediate products such as sodium tetra- 
thionate and chromium dioxide are known to be formed. 
The goods change from yellow to dark brown, then to 
green, and finally to the familiar blue. The sulphur 
makes the final colour a lighter blue than in the case of 
a one-bath tannage, hence the two-bath process is often 
preferred for " colours." 

On account of the empirical character of this " hypo 
bath," it is impossible to fix any exact relation between 
the quantities of material used in the chroming bath, and 
the quantities of " hypo " and acid used in the reducing 
bath. The following rules, therefore, must be understood 
as rough approximations for practical use, and though 
they have been empirically discovered their theoretical 
significance is often fairly obvious. 

1. The amount of hypo necessary is almost directly 
proportional to the amount of dichromate used. In 
chroming with baths of the acid or neutral type, the per- 
centage of hypo should be about three times the percentage 
of dichromate used. Thus 4 per cent, dichromate needs 
12 per cent, hypo ; and 6 per cent, dichromate needs 18 
per cent, hypo on the pelt weight. In baths of the Schultz 
type a less proportion of hypo may suffice, but the 10 per 
cent, hypo for 5 per cent, dichromate, recommended 
by the Schultz patent, is generally considered rather 
insufficient. 



148 



ANIMAL PROTEINS 



2. The proportion of hypo is increased somewhat for 
the heavier classes of goods, and may even reach 20 per 
cent, of the pelt weight. 

3. An increase in the proportion of hypo is usual with 
an increase in the amount of free acid in an acid chroming 
bath. 

4. The percentage of hydrochloric acid in the reducing 
bath is roughly half that of the hypo, but is the most 
variable factor. The quantity varies with the rate and 
mode of addition, the class of goods under treatment, 
and the composition of the chroming bath. 

5. In baths of the Schultz and neutral type it is better 
to add some acid to the hypo bath before adding the goods, 
but this is less essential for goods from an acid chroming 
bath. 

6. In the case of goods from acid chroming baths, 
the amount of acid used in the reducing bath is an inverse 
function of the excess of acid in the first bath, e.g. take 
the following two processes : — 



Chroming bath. 


Hypo bath. 


Dichromate. 


Hydrochloric acid. 


Hypo. 


Hydrochloric acid. 


4 
4 


4 
15 


12 
15 


5 

1 



7. There should be some excess of hypo at the end of 
the process. This acts as a feeble alkali, and commences 
the neutralization. 

The process can be carried out in paddles or in drums 
as preferred, for reasons similar to those applicable in the 
case of the first bath. On the whole, however, drums are 
less popular for the second bath, for the dilute solutions 
of the paddle effect some economy of sulphurous acid, 
which is apt to escape into the air. A preliminary " hypo 
dip " is sometimes used to prevent the " bleeding " of 
the chromic acid. The use of many other reducing agents 



CHROME LEATHER MANUFACTURE 149 

has been suggested as substitutes for hypo. Sulphides, 
sulphuretted hydrogen, polysulphides, sulphites, bisulphites, 
hydrogen peroxide, nitrous acid, lactic acid, etc., have 
been used, but none are so easy to manipulate as thio- 
sulphate. 

Types of One-bath Chrome Tannage. — The one-bath 
process is simpler than the two-bath process inasmuch as 
only one kind of liquor is involved, viz. one in which the 
chromium is in the chromic state. Hence the variants of 
the one-bath process consist mainly of variations in the 
composition of this liquor. The chief point of variation 
is in the readiness with which chromium hydrate is adsorbed. 
This is determined by the extent to which the chromic 
salt is hydrolyzed to form the tanning sol and free acid, 
and by the concentration and nature of this free acid as 
well as of other substances. It is difficult unfortunately 
to express these factors in terms which are comparable 
under general conditions. Chromic salts are usually hydro- 
lyzed to some extent, but this extent is very different even 
in water, according to the nature of the acid radicle. The 
degree of hydrolysis is also largely affected by the extent 
to which the solution has been " made basic " by the 
addition of alkalies. By the neutralization of the free 
acid in this way there is further hydrolysis, the extent of 
which is again influenced by the nature of the acid radicle 
involved and other dissolved substances, especially of 
organic matters. Again, the hydrolysis is largely affected 
by the concentration of the solution even when the pro- 
portions of the ingredients are constant, and this is practi- 
cally important on account of the necessity for exhausting 
the chrome liquors economically. Nor is the matter 
entirely one of degree of hydrolysis, for (as we have noted 
in the preceding section) the electrical condition of the 
chroming sol is of great importance owing to the operation 
of the valency rule and the possibility of zones of non- 
precipitation. The alkaline, neutral or acid condition of 
the goods when first introduced has also its influence on 
all these points. 



150 ANIMAL PROTEINS 

It will be readily understood, therefore, that there 
is some difficulty in expressing the tanning power of a 
chrome liquor. As near as can be yet said this is deter- 
mined by (i) the concentration of the actual tanning sol, 
and (2) its nearness to the isoelectric point. Now, these 
points are not readily determined by analytical methods, 
and the best that can yet be done is to determine the 
conditions which have large influence upon these points. 
Thus the degree to which the liquor is " made basic " by 
adding alkali is known, and can be expressed in formulae 
by assuming that the acid neutralized by this alkali is 
replaced in the chrome salt by hydroxy groups. Chromic 
chloride, Cr 2 Cl 6 , with the addition of soda to correspond to 
half the acid formed upon complete hydrolysis, would be 
considered then to be a solution of the salt, Cr 2 (OH) 3 Cl 3 . 
This has given rise to the conception of the " basicity " 
of a chrome liquor, which may be expressed in many ways, 
the most common of which in practice is the number of 
grams S0 4 still combined with 52 grams Cr. Thus the 
salt corresponding to the composition Cr(OH)S0 4 is said 
to have a basicity of 96. The practical importance of such 
determinations of basicity has been much exaggerated, 
for they are but a rough guide to the degree of hydrolysis 
of the chrome and to the extent to which the sol is positive. 
Thus if the chrome salt be actually a sulphate, a liquor 
of basicity 96 has about the same practical value as a 
chloride liquor of basicity 72, and in each case the figures 
are of little significance if many organic substances be 
present. If, however, as is usual in practice, there be 
approximately the same acid radicles throughout the 
tannage and about the same relative proportion of organic 
matters or of inorganic salts, then these determinations 
have some practical value for comparative purposes. 
The determination is itself simple : a portion of liquor 
is titrated direct with caustic soda. The titration is at 
boiling-point, and is continued until a permanent pink 
is obtained with phenolphthalein. The amount of S0 4 
corresponding to the soda required is then relative to the 



CHROME LEATHER MANUFACTURE 151 

amount of Cr in the same volume of liquor. A chromium 
estimation is therefore also necessary and is most readily 
done by evaporating a portion of liquor to dryness, igniting 
the residue and oxidizing the chrome to chromate by 
heating in a muffle furnace with magnesia and sodium 
carbonate in equal parts, or fusing in a blowpipe with 
sodium and potassium carbonates in equal parts. The 
oxidized residue is dissolved in hydrochloric acid and 
titrated with thiosulphate as described for the two-bath 
process. 

Another attempt to determine the practical value of 
a chrome liquor is the empirical test suggested by 
McCandlish, in which 10 c.c. of the liquor is titrated with 
standard alkali until the precipitation point is reached and 
a turbidity appears. The figure thus indicates approxi- 
mately the degree of nearness to the precipitation point 
and the amount of free acid in the liquor. The author 
has found this a useful test taken in conjunction with the 
basicity determination. It is best expressed in the same 
units, e.g. grams S0 4 per 52 grams Cr. 

Another method is the determination of the hydrion 
concentration of the liquor. This has useful possibilities 
for research work, but is usually too laborious for rapid 
commercial control. The results, moreover, are not less 
empirical, for the hydrion concentration of the liquor 
indicates but imperfectly the electrical condition of the 
particles of the tanning sol. 

In classifying one-bath liquors into types, it is best 
to take together those in which the usual " basicity " and 
" acidity " determinations have at any rate approximate 
comparative value, and this is determined in the main 
by the method by which the liquor is manufactured. 
Broadly speaking, there are three types of chrome liquor : 
(1) those made from chromic salts by adding suitable 
amounts of alkali ; (2) those made from sodium dichromate 
by reduction with organic matter ; and (3) those made 
from sodium dichromate by reduction with sulphurous 
acid or its salts. 



152 ANIMAL PROTEINS 

Of the first type the most common is that in which 
chrome alum (a bye-product of the dyeing industry) is 
the starting-point. To a solution of this a solution of 
washing soda is gradually added, with constant stirring, 
until the salt corresponding with the formula Cr(OH)S0 4 
is obtained. 

Now : — 

K 2 S0 4 Cr 2 (S0 4 ) 3 24H 2 03 + Na 2 S0 4 + NaC0 23 , ioH 2 

998 286 

= 2Cr(OH)S0 4 + K 2 S0 4 + C0 2 + 33H 2 

Hence, in practice, for every ten parts of chrome alum 
2 - 86 parts of soda crystals (or 1*06 parts anhydrous soda) 
are used. A convenient " stock solution " is of 10 per 
cent, strength. Thus 10 lbs. of chrome alum is dissolved, 
made basic, and made up to 10 gallons. To dissolve the 
alum a mechanical stirrer is necessary, for the water must 
not be more than warm. The disadvantage of this liquor 
is the limited solubility of chrome alum and the need for 
its solution in the cold. Iyiquors may be also made by 
dissolving chromium hydrate in hydrochloric acid, and 
making basic to correspond to the formula Cr 2 (OH) 3 Cl 3 . 
Many preparations are on the market containing both 
chlorides and sulphates with appropriate basicity. Chrome 
alum liquors have been less often used in Britain of recent 
years owing to the high price of chrome alum, caused in 
part by the presence in the salt of potassium, all the salts 
of which have been scarce and dear under war conditions. 

Of the second type Procter's " glucose Hquor " is a 
good example. Use 5 lbs. sulphuric acid, 6 lbs. sodium 
dichromate, and 7 lbs. of glucose, or quantities in similar 
proportion. The dichromate is first dissolved, and the 
acid added gradually. The glucose is then added cautiously 
on account of the brisk effervescence of carbon dioxide. 
A glucose of good quality is necessary, and the proportion 
to be used is not quite definite, for sufficient only is needed 
to effect the reduction, and this amount is influenced by 
the rate of addition and temperature of the mixture. The 



CHROME LEATHER MANUFACTURE 153 

reduction should be careful and regular, or the oxidation 
products will be irregular and have a varying effect upon 
the tanning. Molasses can be substituted for glucose, in 
amounts varying with its strength. 

Of the third type the most common is that in which 
the dichromate is reduced by sulphuric acid and sodium 
bisulphite. Solid bisulphite may be used, but it is usually 
dear, and solutions are more commonly employed. Into 
this type fall also the liquors formed by passing sulphur 
dioxide gas into dichromate solution. Stock liquors of 
this type have the advantage that strong solutions may 
be made (up to 18 per cent. Cr 2 3 ) ; they have the dis- 
advantage that they are liable to contain excess of free 
sulphurous acid. 

The method of application of chrome liquors is usually 
by paddling or drumming the goods in solutions of appro- 
priate strength — broadly speaking, paddles used for lighter 
goods and plain finishes, and slowly revolving drums for 
heavier hides and grained finishes. Heavy chrome leather 
is often tanned in pits by suspension just as in vegetable 
tanning. In such instances rockers may be usefully 
employed. 

In any case, the goods are successively brought into 
contact with liquors of increasing strength, as in vegetable 
tannage, and the liquors are thus most conveniently 
exhausted economically. The green goods thus receive 
first nearly spent liquor and finish out of fresh strong 
liquor. The goods may be, of course, handled from drum 
to drum, or from pit to pit, but the modern tendency is 
to save labour by moving the liquors instead. Thus in 
drum tanning the liquor is run out and pumped into the 
next drum. In pits air ejectors have proved suitable, 
not only as lift pumps, but also as agitators of the liquor 
in which goods are suspended. The press system is also used. 

Finishing Operations. — In nearly all cases the chrome 
leather has to be " neutralized " after tanning. This 
consists in removing the acid " reversibly adsorbed." This 
removal is necessary to the finishing processes, as well 



154 ANIMAL PROTEINS 

as to bring the tanning sol into condition for more per- 
manent tannage. Neutralization gets rid of soluble chrome 
salts as well as free mineral acid, and is the final stage in 
rendering the tanning sol less positive, and perhaps even 
negative. It is brought about by the use of weak alkalies, 
of which borax is the easiest and safest, but not the 
cheapest. Sodium silicate, phosphate, carbonate, and 
bicarbonate have been also used, and a mixture of soda 
and an ammonium salt has been suggested by Stiasny. 
Whitening has also been tried, but is very slow- acting. 
Considerable economy in alkali may be effected by a 
thorough washing of the leather before using the alkali. 
If the water be hard, so much the better, and if warm 
water be available the process is hastened. For most 
leathers it is necessary to remove excess of alkali just as 
much as excess of acid, so that a thorough washing in 
water generally follows the treatment with alkali. Any- 
thing from | to 3 per cent, borax (or its equivalent) on 
the pelt weight may be used, and, generally speaking, it 
is better to use solutions as dilute as practicable in order 
to avoid local over-neutralization and tender leather. 

Fat liquoring is a process which is very largely typical 
of chrome leather manufacture ; it consists in drumming 
the goods with an oil emulsion, the grease of which is 
entirely taken up by the leather. It thus strongly resembles 
drum stuffing (Part I., Section IV., p. 53) in method, but 
the " fat liquor " is such that it mixes easily with water, 
and usually contains soap in order to assist in this sense, 
and may sometimes indeed consist of soap only. Mineral 
oil is also used frequently in fat liquors. The object of 
fat liquoring is to give softness, pliability, or waterproofness, 
and to feed the " empty " chrome tannage. It is also 
used as a preparation for more complete impregnation of 
grease, e.g. as in " stuffing " chrome harness, and in 
" dipping " chrome sole leather. Fat liquors are usually 
made by dissolving the soap in boiling water and gradually 
adding the oil with constant agitation. Perfect emulsifi- 
cation is essential, and this is assisted by the use of casein, 



CHROME LEATHER MANUFACTURE 155 

albumen, gelatine, starch, egg yolk in addition to soap 
and oil. Soda and borax also assist, and degras and sod 
oil are also useful and are admissible where the leather is 
to receive a dull finish. The operation of fat liquoring is 
greatly assisted by heat, and temperatures of about no 
to 130 F. are usual. Chrome leather may be dyed before 
or after fat liquoring : if before, the fat liquor sometimes 
tends to alter the shade ; if after, the dyeing tends to be 
uneven. IyOgwood extract and iron salts are largely used 
for blacks. It is common to mordant chrome leather with 
vegetable tanning before dyeing. Sumach and gambier 
are often used for this purpose, and the usual " fixing 
agents " (tartar emetic, titanium salts, etc.) may also be 
used. 

Of the mechanical finishing operations staking is the 
most characteristic. It is now done entirely by machines, 
and the primary purpose is to soften the leather, which 
otherwise dries out in a non-pliant and stiff condition. 
In the staking machine, the " blade " is fixed between 
two rollers, which are however on the other side of the 
leather. The leather is held by the operator, and the 
machine " head " pulls a fold of the leather over the blade. 
Seasoning and glazing are also common for many chrome 
leathers. 

REFERENCES. 

Procter, "Principles of Leather Manufacture," pp. 198-220. 
Bennett, "Manufacture of Leather," pp. 210, 312, 355, 375. 
Bennett, "Types of Two-bath Chrome Tannage," Leather, 1909, 
Aug. and Sept. 



Section III.— CHROME CALF 

The tannage of calfskins by the chrome processes for the 
manufacture of upper leathers is one of the most extensive 
branches of leather manufacture. The deservedly popular 
box calf is typical of these leathers, and the observations 
of this section are primarily applicable to it. A chrome - 
tanned calf skin, fat liquored and blacked, provides as 
suitable an upper leather as could be desired for ordinary 
boots. It is at once supple and durable. It is also suffi- 
ciently waterproof, but can be given a bright glazed finish. 

In regard to the wet work for chrome calf, the general 
principles and methods discussed in the previous section 
are much to the point. It is essential to avoid undue 
plumping and the loss of hide substance. The skins should 
be washed clean as soon as possible. Three fresh waters 
are desirable, the goods remaining only a short time in 
each. Salted skins need more time, but the liquors must 
be kept sweet. Drumming the skins in running water is 
very suitable for the first and last stages of soaking. 

The liming should be short but not "sharp," i.e. 
mellow sulphide limes are suitable, depilation being carried 
out after about 7 days. The one-pit system is usual, but 
two liquors may be given, the green goods being first 
inserted into a used liquor, and after handling re-inserted 
into the same pit with a new lime liquor made up with 
lime, sulphide and a proportion of the old liquor. Scudding 
should be carefully done, as hair on the finished leather is 
very objectionable. 

In deliming it is essential to have the grain of the 
skins thoroughly relaxed and pulled down. The finished 
box calf should have a characteristic soft and silky feel, 
and this is only attained by procuring the inelastic pelt. 



CHROME CALF 157 

It is not surprising that a light puering is a popular method 
for attaining this, but there is also a tendency to use 
artificial bates such as are made from ammonium chloride 
and pancreatin, together with organic acids, or non-swelling 
acids like boric acid. Drenching is also common after a 
preliminary deliming with acid. The skins may be half 
or two-thirds delimed with lactic acid, rinsed and drenched 
over night at 85 F. with 6 per cent, bran on the pelt 
weight. Iyess acid may be also used, in tepid water, and 
the drench made up with 10 per cent, bran and a little 
pea meal. It is very common to pickle the skins in 5 per 
cent, alum and 5 to 10 per cent, salt before tanning. This 
is often of doubtful advantage, but sometimes prevents 
drawn grain when the goods are moved rapidly into strong 
chrome liquors. This pickling is said to give fullness to the 
leather. 

The tannage of box calf is usually by the one-bath 
process, though the two-bath process gives quite as good 
a result and is sometimes used. Again, drum tannages 
are the most popular on account of their speed and the 
economy of chrome. The practical problem is to use up 
all the chrome, and to tan quickly without " drawing " 
the goods. It is, in any case, usual to commence the 
tannage in a used and nearly spent liquor and finish in 
a fresh liquor. The most appropriate way depends largely 
upon local convenience, the number of drums available, 
supply of labour, etc. In a one-drum system the goods 
may be started in an old liquor, which is run off when 
exhausted by the green goods. Fresh stock solution is 
then added at intervals of an hour or two and the drumming 
continued till tannage is complete, which is usually in 
less than 24 hours. The remaining liquor is used to 
commence the tannage of the next pack. 

In another system the operation is similar except that 
the liquors are weaker, and the goods are then removed 
and finished in another drum. A three-liquor system, 
however, is often combined with a one-drum method ; 
the goods are thus not handled. The liquors are run off 



158 ANIMAL PROTEINS 

and pumped to other drums, the once-used liquor to a 
drum containing goods already treated with a twice-used 
liquor ; the twice-used liquor to a drum containing green 
goods, and the thrice-used liquor pumped to the drain. 

In any of these methods the chrome alum liquor is 
suitable, using 10 per cent, alum and 3 per cent, soda on 
the pelt weight. The glucose liquor has also proved very 
suitable for chrome calf, and the liquors made with sul- 
phurous acid or its salts have increasing popularity on 
account of lower costs. Many tanners use bought liquors 
— " chrome extracts," which are supposed to be specially 
devised to suit the tannage of chrome calf. When 
thoroughly tanned through, as can be readily judged from 
a sectional cut of the leather, and also by the strength of 
the liquor remaining, the goods are horsed in pelt overnight, 
and are then ready for finishing. 

In finishing box calf the neutralization should be 
thorough, or the acid may cause trouble in dyeing and 
fat liquoring. Imperfect removal of excess chrome salts 
may cause the formation of " chrome soaps," which are 
very difficult to remove ; the goods should therefore be 
well washed. There are two general types of treatment 
before blacking. In one, the skins are first well washed 
with water at no° F., neutralized with about 3 per cent, 
borax, and well washed again. Striking follows and is 
usually very thorough, partly because it assists in pro- 
ducing evenly the characteristic box grain, and partly 
because the finished leather is sold by the square foot. 
Machine striking is now almost universal, and may be done 
several times at different stages in the drying. When 
half dry (" sammed ") the skins are shaved by machine 
and, at this stage usually, weighed. Dyeing and fat 
liquoring then follows. In the other type, the goods are 
merely washed, and then struck out, sammed, shaved and 
weighed. The skins are then neutralized, washed and 
immediately dyed and fatliquored. The advantages of 
this latter course are that the goods remain in the drum 
for the last four processes, which is economical of labour, 



CHROME CALF 159 

and also that by neutralizing immediately before dyeing 
and fat liquoring there is less danger of a further diffusion 
of acid. 

In dyeing logwood extract is largely used, occasionally 
a little fustic is used also, and by using a " striker " of 
iron and copper sulphates a good black is obtained. IyOg- 
wood is often used also in conjunction with coal-tar 
dyestuffs. The goods are first warmed in the drum up 
to 140 F., and the dyestuff solution gradually run into the 
drum whilst it is revolving. Up to § hour may be necessary 
to exhaust the bath, the goods being constantly drummed. 
The fat liquor is then run in similarly, and the drumming 
continued until the grease is all absorbed by the leather, 
which may take another hour. The skins are horsed till 
next day, during which time the grease penetrates more 
completely. 

The skins are now dried out, sometimes by suspending 
from the hind shanks and sometimes by nailing on boards 
or wooden frames. They are damped back for staking 
by leaving for i| to 2 days in moist sawdust. After 
staking they are dried strained in a" stove " at about 
105° F. 

In finishing off, the grain is " cleared " by sponging 
with 10 per cent, lactic acid, and seasoned with a mixture 
* of milk, blood and black dyestuff. When dry on the 
surface the skins are glazed by machine, and grained two 
ways — neck to butt and belly to belly. They are usually 
reseasoned, dried out, reglazed, regrained, lightly oiled 
with mineral oil, and finally trimmed. These various 
operations are fairly typical, but there is obviously ample 
scope for divergence. Thus one may fatliquor before 
dyeing, and the skins may be staked before drying out, 
and may be restaked after glazing. 

Much so-called " box calf " is not made from calf skins. 
A very close approximation, however, is obtained from 
rather older animals, and " box-kip " is largely manu- 
factured by similar methods. Light hides are also widely 
used, being similarly treated except that they are split 



160 ANIMAL PROTEINS 

and also cut into two along the spine. The finished article 
is sold as "box-sides." To yield the characteristic grain 
pattern, the goods are frequently printed and embossed. 
Even the flesh splits are sometimes made into box calf 
imitations, some filling material being used and an artificial 
grain pattern embossed. 

Willow calf typifies the chrome calf which is finished 
in colours. The soaking, liming and deliming processes 
are the same as for box calf. The tannage, however, is 
generally by the two-bath process on account of the lighter 
colour thereby obtained. This colour is largely due to 
the deposition of sulphur in and on the leather in the 
second bath. 

In one tanning process the skins are first pickled in 
2 per cent, hydrochloric acid and 10 per cent. salt. They 
are then drummed in solution containing 2 per cent, 
dichromate (strength 1 in 60) for about half an hour. A 
solution containing 4 per cent, dichromate, 3! per cent, 
hydrochloric acid, and 5 per cent, salt is gradually added, 
and the skins drummed until well struck through. They 
are then horsed overnight and struck out and passed 
through a " hypo dip," — a 2 per cent, solution of thio- 
sulphate, — and then into the reducing bath, which contains 
10 per cent, of thiosulphate, to which 5 per cent, hydro- 
chloric acid is added. 

Another process employs paddles instead of drums. 
The chroming liquor is made up with 4! per cent, chromic 
acid and 10 per cent. salt. The bath is exhausted by 
commencing the tannage of a succeeding pack. The skins 
are reduced as in the last process. 

In another process the " acid " type of chroming bath 
is used. The skins are paddled with a solution containing 
5 per cent, dichromate, 5 per cent, hydrochloric acid, 2 
per cent, aluminium sulphate, and 10 per cent. salt. In 
the reducing bath 14 per cent, hypo and 4 per cent, hydro- 
chloric acid are used. 

In yet another process the skins are pickled first in 
5 per cent, aluminium sulphate, 7! per cent, salt, and 



CHROME CALF 161 

3 per cent, sulphuric acid, and are then dried out and 
sorted. The tannage proper is in the drum, using 6 per 
cent, dichromate, 5 per cent, hydrochloric acid, and 5 
per cent. salt. In the reducing drum 15 per cent, hypo 
is used and 4§ per cent, hydrochloric acid. 

Whichever process of tanning has been used, the skins 
are neutralized and washed thoroughly, as for box calf, 
sammed and shaved. In dyeing, the skins are first 
mordanted with a filtered infusion of leaf sumach, used 
at no F. for half an hour. As fixing agent, 4 oz. 
tartar emetic per dozen skins is then added and the drum- 
ming continued for half an hour. The goods are washed, 
struck out and drum dyed at 140 F. with basic colours, 
and immediately fat liquored. In the fat liquors olive 
oil and castor oil, with the corresponding soaps, have been 
popular, but substitutes are now used on economical 
grounds. The skins are next horsed a while, well struck 
out again and dried strained. They are now finished off 
as for box calf, except that it is usual to grain only one 
way — neck to butt — and the season should consist of milk, 
water and albumin only, though sometimes other muci- 
lagenous matters are added. As with box calf, the finishing 
may be varied in many ways. The skins may be dyed 
with acid colours after fat liquoring. For pale shades 
direct dyes are used without a mordant. For darker 
shades of brown and red, the dyewoods are used both as 
mordants and ground colours, and titanium salts are useful 
as fixing agents. 

Both the " box " and " willow " finish are largely a 
matter of public taste, and the fashion varies from time 
to time on such points as to whether the grain should be 
one way or two ways, and whether it should be faint or 
bold. There are also other common finishes besides the 
typical box grain. Glace calf is made much in the same 
way as box calf, but there is no graining at all. The goods 
are usually seasoned and glazed three times. Small skins 
are preferred for this finish. Dull calf is also a plain 
finish. The leather contains more grease, and the fat 
K. II 



162 ANIMAL PROTEINS 

liquor is made up with greater proportions of degras. The 
goods are not seasoned or glazed, but ironed, " sized " with 
gum, oil, soap and logwood, and after brushing are dried 
and rolled. In both these plain finishes a one-bath paddle 
or pit tannage is common, in order to ensure the smooth 
finish. 



REFERENCES. 

Procter, " Principles of Leather Manufacture," p. 198. 
Bennett, "Manufacture of Leather," pp. 55, 84, 105, 227, 360-363, 375. 
Bennett, "Theory and Practice in Wetwork of Chrome Calf," Shoe and 
Leather Reporter, Sept., 1909. 



Section IV.— CHROME GOAT AND SHEEP 

Immense quantities of goat and sheep skins are chrome 
tanned for upper leathers. Most of them are manufactured 
into the well-known and popular Glace kid, to the manu- 
facture of which this section is chiefly devoted. To be 
quite strict, glace kid should be made from kid skins, but 
actually comparatively few of such skins are used, they 
being reserved rather for glove leathers. The popular 
upper leather is made from goatskins. 

Chrome goat is deservedly popular ; it is an ideal upper 
leather for shoes and light boots. As compared with 
chrome calf (thickness and other factors being equal), it 
is not only softer and more pliant, but also more durable. 
It is usually, however, not quite so thick, and perhaps 
therefore not quite so warm and waterproof. The popu- 
larity of glace is probabl3 r enhanced by the brighter and 
more glassy finish than is usual with box. 

As the supply of goatskins is unfortunately too limited, 
an even more widely used glace upper leather is made 
from sheepskins, and often sold as glace kid. From what 
has been previously said as to the quality of goat and 
sheepskin leathers (Part II., Sections II. and IV.), it will 
be readily understood that glace sheep is by no means 
so good a leather as glace goat. It is perhaps as soft, but 
is more spongy and loose textured, and is neither so water- 
proof nor so durable as chrome goat. The ubiquitous 
sheep, however, provides an immense supply of raw 
material, and the resulting leather, which should strictly 
be regarded as a glace kid imitation, finds a ready sale. 
When well finished it is indeed a good imitation in respect 
of appearance, and this fact, together with its comparatively 
low cost, causes it to meet an undoubted public need. 

The production of glace goat will first be considered. 



164 ANIMAL PROTEINS 

The soaking process is quite similar to that before described 
for the production of goatskin moroccos (q.v.) and need 
not be here repeated. The liming is similar in many 
respects also, but from what was said in Section II. about 
the undesirability of excessive plumping of pelt for chrome 
leather, it will be clear that caustic soda should be omitted 
from the limes. The liming should also be shorter for 
glace than for moroccos, and this is attained both by using 
a greater proportion of sulphide and by using mellower 
lime liquors, preferably the latter, as soft pelts are better 
ensured. Calcium chloride has sometimes been added to 
the limes : this reacts with the soda from the sulphide, 
yielding salt and probably precipitating lime, and has its 
own lyotrope influence, thus reducing the plumping effect 
possibly in two ways. To obtain either effect it is 
necessary to use considerable amounts of calcium 
chloride. As goatskins are so tight fibred, a longer 
liming and a greater loss of collagen is permissible than 
with most pelts for chrome. The deliming operations 
should be exceedingly thorough in order to obtain the 
desired softness and the smooth grain. Puering is largely 
used to the full extent, i.e. the goods are thoroughly pulled 
down at 85°-o,o° F., and are carefully delimed in the 
puer liquor. After puering it is common to give a low 
temperature drench (6o°-65° F.), which of course acts 
slowly over a day or two. The skins must be well scudded 
after puering or after drenching ; sometimes after both. 
The drenching is often substituted for purely deliming 
processes, of which may be mentioned the use of boric 
acid and also the use of warm solutions of the commercial 
organic acids (lactic, formic, acetic, butyric, etc.), together 
with calcium chloride. In place of the chloride, a salt 
of the acid may be emp^ed, and the deliming bath may 
be regenerated by oxalic acid and used repeatedly. Some- 
times puering is omitted and the desired result obtained by 
washing in warm water, nearly deliming with warm solu- 
tions of organic acid, washing again and drenching. Skins 
are also washed often after drenching. 



CHROME GOAT AND [SHEEP 165 

In tanning chrome goat for glace the two-bath process 
is mostly preferred. This is partly because the sulphur 
deposited in the reducing bath assists materially in pro- 
ducing the mellowness and fullness which are so essential, 
and partly because a large proportion of skins are finished 
in colours. The two-bath process also lends itself to a 
paddle tannage, which is necessary for the smooth grain 
finish. One or two illustrative processes may be given. 

One process presents many points of resemblance to the 
first process suggested for willow calf in Section III. [q.v.). 
The skins are first pickled in a paddle with 2 per cent, 
hydrochloric acid and 10 per cent, salt, and then pass into 
the chroming paddle, which contains at first only 2 per 
cent, dichromate. Subsequently 4 per cent, dichromate, 
3 1 per cent, hydrochloric acid, and 5 per cent, salt are 
added to the paddle liquor, and the skins paddled until 
well struck through. After being horsed overnight the 
skins are struck out by machine, passed through a hypo 
dip if desired, and reduced with 12 per cent, of thiosulphate 
and about 5 per cent, of acid. The skins may be left 
overnight in the hypo paddle, and the excess of thiosul- 
phate, which is a feeble alkali, commences the neutralization. 

In another process the chroming bath is made up of 
5| per cent, chromic acid and 6| per cent, of salt, and to 
this paddle liquor 2 or 3 per cent, of aluminium sulphate 
may be added if desired. The reduction is with 14 per 
cent, hypo and 7 per cent, hydrochloric acid. A little 
of the acid is added to the reducing bath ; when the liquor 
turns milky, the skins are rapidly inserted, and the rest 
of the acid gradually added. 

In the finishing processes the mechanical operation of 
" striking " is very prominent, on account of the necessity 
of obtaining area and smooth grain. The skin of goats 
has rather a tendency to bold grain, and this enhances 
the need of striking. Most manufacturers lay great stress 
upon thorough neutralization and washing. An important 
point also is that the staking should be carried out at the 
proper condition of dryness. If either too damp or too 



166 ANIMAL PROTEINS 

dry, the requisite mellow feel is not obtained. There is, 
of course, ample scope for variation and ingenuity, and 
the following processes for blacks and colours must be 
taken as broadly typical. 

The skins from the reducing bath are first machine- 
struck, and then immediately neutralized with one per 
cent, borax until this is thoroughly used up, and the skins 
are then paddled for many hours in running water. They 
are again struck out and lightly shaved, possibly after a 
little drying. There is a tendency to save time by using 
a stronger borax solution, and by using warm or tepid 
water, and some factories save borax by washing well 
first in warm water. If for blacks a common plan is to 
dye grain and flesh a violet -blue and then black the grain 
only with logwood and iron. The skins are drum dyed 
blue with a coal-tar dyestuff, drumming half an hour in 
the solution at no° F., and again struck out. They are 
then paired or pleated, and rapidly passed successively 
through three vats containing respectively cold weak 
ammonia, a logwood and fustic infusion at 120 F., and 
a solution of ferrous sulphate containing a little copper 
sulphate. The skins must be immediately washed well 
to remove excess of iron. Instead of this process the skins 
may be passed through vats containing coal-tar blacks. 
Instead of blue backing the skins may be drum-dyed black 
on flesh and grain with either coal-tar blacks or with 
logwood and iron. In the latter case the skins must be 
drummed in water for an hour to remove excess of iron. 
However dyed, the skins are often struck out again after 
dyeing, and sammed slightly for fat liquoring. Neatsfoot 
oil is a popular ingredient of the fat liquor. The skins are 
drummed dry for a few minutes in a hot drum, and the 
fat liquor added at 130 F., and the drumming continued 
after the grease has been taken up in order that it may 
be thoroughly distributed. The skins are struck out again, 
rapidly dried out, and wet back for staking in damp saw- 
dust. The staking should be thorough, and, if necessary, 
repeated when the goods are rather drier. 



CHROME GOAT AND SHEEP 167 

In finishing off the skins may be fluffed if desired, and 
are then " cleared " by sponging with 10 per cent, lactic 
or acetic acid. They are then seasoned and glazed after 
some drying. This is repeated until the required gloss 
has been obtained. They are finally oiled lightly with a 
mixture of linseed and mineral oils. On finishing dull 
kid a heavier fat liquor is given, in which degras is used, 
and the skins are not seasoned and glazed, but are ironed 
and oiled. In finishing for coloured glace, the skins are 
mordanted before dyeing by the use of dyewood extracts, 
antimony and titanium salts being used as fixing agents. 
The fat liquor should contain less soap and more egg yolk, 
and for fancy shades even egg yolk only is sometimes used. 

The production of chrome glace sheep follows the same 
general lines as glace goat. There is less difficulty in 
obtaining smooth grain, so that " striking " is perhaps 
less prominent, and drum tannages are preferred, whether 
one bath or two bath. The skins are received after fell- 
mongering (see Part II., Section IV.) and need thorough 
puering to remove scud, and may be then rinsed through 
boric acid. Pickling is very common with these goods. 
In the pickled state they are often sorted out before 
tanning. The pickling is usually a one-bath process in 
which vitriol and salt or else alum and salt are used, but 
sometimes all three substances. The skins may indeed 
be received in a pickled state. They may be depickled 
by paddling with salt and borax, bicarbonate, or basic 
alum solution. They may also be tanned without de- 
pickling if the composition of the pickle be allowed for in 
the first chroming liquor. A commonly used pickle consists 
of 3 per cent, aluminium sulphate and 9 per cent. salt. 
If these goods are to be dried out, flour also may be used 
with the pickle, which thus becomes practically a light 
preliminary alum tannage (see Part IV., Section I.). A 
commonly used acid pickle is of 5 per cent, commercial 
sulphuric acid and 25 per cent. salt. 

The delimed or depickled stock may be tanned as now 
described. The two-bath process may be used with drums. 



168 ANIMAL PROTEINS 

The chroming bath contains 5 per cent, dichromate, 5 
per cent, hydrochloric acid, and 10 per cent. salt. After 
the skins are thoroughly penetrated they are horsed over- 
night and reduced with 20 per cent, thiosulphate, up to 
7 per cent, of hydrochloric acid being added after half 
an hour in thiosulphate only. 

Alum pickled or tawed skins are wet back by drum- 
ming for about an hour in water, and are then tanned by 
the one-bath process in drums. Only a few hours are 
needed. Towards the end of the operation about \ per 
cent, of bicarbonate of soda may be added to the chrome 
liquor. Acid pickled skins may be wet back with 10 per 
cent, salt, and' depickled by adding a basic alum solution 
and the chrome tannage superimposed after about half 
an hour without handling the goods. The basic chrome 
alum liquor is suitable for this purpose. 

In finishing glace sheep much the same methods are 
used as in the case of glace goat. Sheepskins are perhaps 
more lightly fat liquored, being naturally soft and porous. 
Degreasing is often necessary to obtain an even finish. 
As sheep gives an empty pelt and chrome an empty tannage, 
a slight retannage is often given in gambier, especially for 
blacks, in which case the skins are well mordanted. This 
retannage makes the leather less stretchy. L,ogwood and 
iron blacks are usual. For colours, fustic or sumach are 
the usual mordants, with tartar emetic to fix. If for glove 
leathers, skins pickled in alum and salt or tawed should 
be preferred, and flour may be used in the fat liquor. 

Sheepskin splits are sometimes given a chrome tannage 
and finished as chrome chamois. This leather may be 
used for linings, but not for polishing silver on account 
of the sulphur originating from the reduction bath. The 
splits are puered heavily, and pickled in 6 per cent, vitriol 
and 24 per cent. salt. They are paddled in this pickle 
liquor, and 4 per cent, dichromate added in successive 
portions. The fleshes are horsed overnight and reduced 
in 15 per cent, thiosulphate, to which a little hydrochloric 
acid is added if needed. 



CHROME GOAT AND SHEEP 169 

In finishing the splits are washed in warm water, 
neutralized in weak soda, and washed again. They are 
sammed by machine striking, and fat liquored, using much 
soap. They are then horsed, struck and dried out. They 
are staked several times after damping back, drying out 
again between stakings. They are finally fluffed. 



REFERENCES. 

Procter, " Principles of Leather Manufacture," p. 198. 
Bennett, "Manufacture of Leather," pp. 55, 84, 105, 230, 364. 
Bennett, "Theory and Practice in Wetwork of Chrome Goat," Shoe 
and Leather Reporter, Sept., 1910. 



Section V.— HEAVY CHROME LEATHERS 

The term " heavy chrome leather " is taken to include 
chrome sole leather, chrome strap and harness butts, water- 
proof chrome upper leathers, motor butts and picking 
band butts. These will be discussed in turn. 

Chrome sole leather, as stated in Section I., has made 
headway in Britain during the European War, the Army 
authorities having recognized its great advantages in 
durability and waterproof ness. At the time of writing, 
however, its manufacture has received a set back, and 
many factories are reducing their output. The primary 
cause of this is that the Army purchases have largely 
ceased, whilst the general public have not yet been educated 
to its value. Men who take chrome uppers for granted 
talk of chrome sole as a " leather substitute " with an 
implication that it is of inferior value. It must be recog- 
nized, too, that there is some interested opposition to its 
development. Cobblers and bootmakers complain that it 
ruins their tools, being so hard to cut. Now, it is manifestly 
impossible for it to be soft to cut and hard to wear out ; 
the complaint is therefore an excellent testimonial. There 
is also a stupid fear that an article which lasts twice as 
long will reduce repairs and retail sales by 50 per cent. 
Even the manufacturer has sometimes a suspicion that a 
demand reduced in proportion to durability will not be 
balanced by an extended export trade. These points of 
view will become minor considerations when the public 
realize its relative economy, and when the community as 
a whole grasp that a durable article is a natural asset. 
Meanwhile credit is due to those firms who persevere in 
their pioneering work of educating the public. 

The manufacture of chrome sole leather presents many 
analogies with the vegetable tannages. The soaking and 



HEAVY CHROME LEATHERS 171 

liming should be about identical, but the hides for chrome 
are generally given more sulphide and the depilation is 
reduced to about a week. The methods used for deliming 
differ widely in different factories. Some delime com- 
pletely with mineral acids, some even pickle in acid and 
salt, whilst others merely delime the grain with boric acid. 
The last is really quite sufficient. Again, in tanning one 
finds similar divergences of method. Drum tanning is 
practised, but tannage in pits by suspension is more usual, 
though, as this last involves more dilute liquors, it involves 
also greater time to tan. In drum tannages a few days 
only are sufficient. In pit tanning at least a week is given, 
but sometimes up to a month, according to the strength 
of the final liquor and the rate of progress of the goods 
into stronger liquors. L,iquors containing over 1 per cent. 
of chromium may easily be spent out so as to contain only 
o*oi per cent, labour and time are saved in pit tanning 
by the use of rockers. The press system of avoiding 
handling, however, so complicates the analytical control 
that its advantage is doubtful, a better way being to shift the 
liquors by an air ejector, which may also be used as an 
agitator of the liquor and thus abolish the need for rockers 
("Forsare" patent). Chrome butts are tanned out in 
suspension. No floats or layers are used. The neutralization 
need not be so thorough as for light chrome uppers, as dyeing 
is not practised and trouble does not arise with emulsions 
made from sulphonated oils. Thorough washing is advisable, 
and the butts are usually then cut into bends and may 
be oiled before drying if desired. The bends are dried 
strained, to obtain flatness and smooth grain, for no 
machines, such as strikers and rollers, are usually employed. 
It is necessary to dry very thoroughly, for the bends are 
waterproofed by dipping the dry leather into molten waxes. 
The most commonly used wax and the cheapest is paraffin 
wax with a m.p. of about 127 F. It is rather a brittle 
wax, however, and as the finished leather consists of up 
to one-third of the wax, it is better to use at least some 
proportion of hard fat, Japan wax or ceresin wax, to 



172 ANIMAL PROTEINS 

obtain a stuffing material with less crystalline texture. 
The use of 10-30 per cent, rosin in the stuffing grease 
is also usual. This prevents the leather from being so 
slippery when in wear. The stuffing should take place 
at temperatures from i5o°-i95° F., according to the 
melting-point of the grease employed. The bends are 
taken out and laid in pile to cool and set in a flat condition, 
and are then finished. 

The chrome tannage of butts for strapping and harness 
backs, and for motor butts and picking bands may be 
similar to that for chrome sole, but drum tannages are 
more common and the two-bath process is often used. In 
the latter case the acid chroming bath is preferred, using 
6 per cent, of dichromate and of acid, with up to 15 per 
cent, of salt, and reducing with 15 per cent, thiosulphate 
and acid as needed. This process assists in the production 
of the light colour which is preferred in the case of some 
of these leathers. 

Strap butts after tanning are very thoroughly washed 
with cold water in pits, and repeatedly struck out by 
machine between the washings. They are then oiled with 
heavy mineral oil, and stretched by powerful machines. 
They are dried and curried during the stretching. Degras, 
wool fat and vaseline are greases used, and the drying 
and stretching finished off at 120 F. They are then 
fluffed on the flesh, French-chalked and heavily rolled. 

Harness backs are neutralized, machine sammed, and 
lightly fat liquored with 4! per cent. soap. They are then 
struck and oiled with heavy mineral oil and dried for 
stuffing. Hand stuffing, drum stuffing, and " burning in " 
are all used (see Part I., Section IV.). Stearin, paraffin 
wax, ceresin wax, wool fat, sod oil and mineral oil are the 
greases employed. The butts are blacked after stuffing 
with lamp black and oil, glassed well and buck-tallowed 
on the grain. 

Motor butts are fat liquored lightly, using soap only. 
They have to be softened, therefore, during the drying 
by being mechanically worked. A boarding machine is 



HEAVY CHROME LEATHERS 173 

repeatedly used during the drying. They are finished off 
with French chalk on flesh and grain. 

Picking band butts are neutralized by using warm 
water and then borax solution, and are then sammed by 
machine and very heavily fat liquored with cod oil and 
tallow and hard soap, to which degras may also be added. 
Up to 20 per cent, of greases (on the pelt weight) may be 
used. They are well drummed in this, struck out, French 
chalked, and dried out. They are softened finally by 
machine. 

Waterproof chrome upper leathers are manufactured 
usually from hides tanned by the two-bath process, which 
is said to give a mellower leather. The neutral type of 
chroming bath is common. The butts are neutralized, 
machine sammed and struck, and then fat liquored with 
2 per cent, each of neatsfoot oil and soft soap. The)- are 
then sammed, shaved and blacked on the grain with log- 
wood and iron, and dried further. They are stuffed then 
by brushing with an abundant amount of concentrated 
fat liquor. This gives the waterproofness. They are 
staked after drying further, and often grained three ways. 
A further waterproof finish is given consisting of a fat 
liquor containing bees-wax. They are finally brushed 
and re-oiled with linseed oil, to which some mineral oil 
may be added. This leather is much the most durable 
type for a shooting boot, or where waterproof uppers are 
desirable. 



REFERENCES. 

Procter, " Principles of Leather Manufacture," p. 198. 
Bennett, " Manufatture of Leather," pp. 234, 368. 



Part IV.— MISCELLANEOUS TANNAGES 

Section I.— ALUM TANNAGES 

The use of alum for making pelt into leather is several 
centuries old. It was the first case of what are called 
" mineral tannages." The tannage is closely analogous 
in theory to the chrome tannages discussed in Part III., 
and if soda be added to ordinary potash alum in 
solution, a basic alum liquor is obtained which is 
quite capable of yielding a satisfactory leather, and 
which is thus a strict analogy of the basic chrome alum 
liquor described in Part III., Section II. The range of 
basicity which is practicable is very limited, however, and 
it is much more usual to use common salt with the 
alum instead of soda. The alum is, of course, hydrolyzed 
and free sulphuric acid is quickly adsorbed, whilst the 
colloidal solution of alumina is adsorbed also but more 
slowly. The adsorbed acid tends to swell the pelt and 
to cause it to take up the alumina less readily. The function 
of the salt is to repress the swelling by a pickling action. 
The actual result is thus partly due to the alum tannage 
and partly due to the temporary tannage given by the 
pickle. Hence such tannages are not firmly " fixed," nor 
is the result water-resisting, for much of the tanning 
material may be washed out. If, however, such leathers 
be stored for a time in a dry condition, the alumina becomes 
much more firmly fixed, owing probably to a further 
dehydration of the alumina gel deposited upon the fibres. 
The tannage is thus relatively more " irreversible," and 
such storage is practised in commerce for this purpose, 
being known as the " ageing " of the leather. It will be 
understood that it is possible to use too great a proportion 



ALUM TANNAGES 175 

of salt, the hygroscopic nature of which would keep the 
leather moist and thus interfere with a glossy finish. About 
one-third the weight of the alum used is usually sufficient. 

All that has been said in Part III. as to the empty 
nature of the chrome tannage is equally applicable to the 
alum tannages. It is as necessary therefore to employ 
filling agents. A fat liquor is quite satisfactory for many 
purposes, but is too dark coloured and greasy for glove 
leather. Egg yolk is the favourite emulsion in these cases. 
It contains about 30 per cent, of an oil very similar to 
olein and in very perfect emulsion. Olive oil is also largely 
olein and is also used, being emulsified by the egg yolk and 
effectively reducing the proportion required of this expensive 
material. Flour is also used as a filling agent. It acts also 
as a whitening agent and as an emulsifier. Its use enables 
the tanner to obtain the required fullness without so much 
greasiness. Thus softness and fullness may be obtained, 
and yet a glossy finish be possible. It will be clear that 
the more flour is used, the more oil may also be used. 

The materials mentioned, viz. alum, salt, flour, egg 
yolk and olive oil, are all mixed together into a paste with 
some amount of water. The goods are drummed in this 
paste and then dried out. This operation is known as 
" tawing." The goods are then " aged " for several weeks 
and finished as required. 

The manufacture of " glove kid " from lambskins and 
kid skins is the most typical example of alum tannage. 
Lambskins are unwoolled very usually by painting the 
flesh with a mixture of lime and sodium sulphide. There 
must not be too much of the latter on account of its tendency 
to give harshness, a fatal defect in glove kid. The addition 
of calcium chloride is desirable, and the skins, which should 
be pulled as soon as possible, should be quickly placed in 
soft water or weak lime. For kidskins a set of lime liquors 
may be used, and in preference to sodium sulphide red 
arsenic is employed. About one per cent, realgar on the 
weight of the lime is used, but more often larger quantities 
are preferred, even up to 6 per cent. The liming is thus 



i 7 6 ANIMAL PROTEINS 

shortened to 4 or 5 days. Fresh lime liquors are some- 
times used without any sulphides. Another method is 
to place the skins in a paste of lime to which realgar has 
been added in slaking. In any method it is necessary to 
saponify or emulsify the grease on the grain, or difficulties 
occur in dyeing and finishing. 

Skins which are to be tawed for glove kid are both 
puered and drenched. They are heavily puered at 70 F. 
for 3 hours, or even longer for the heavier skins. After 
scudding they are drenched with 10 per cent, bran and 
some pea meal at 95 F. for a few hours only. 

In preparing the tawing paste, the flour should be 
mixed with tepid water ; the egg yolk should also be 
diluted with tepid water slightly, and strained if necessary, 
and then added to the flour. The oil is then carefully 
mixed in. The alum and salt are dissolved separately at 
110 F. and added to the flour and oil. The tawing paste 
should be used at about 10 5 F. For every hundred 
medium-sized lambskins there will be required : 10 lbs. 
flour in 2 1 gallons water, 1 quart preserved egg yolk, 3f 
lbs. alum and i| lbs. salt. The skins are drummed in this 
for an hour or so and dried out on poles rapidly, but not 
with great heat. This is essential to get " stretch." They 
are next wet back, staked, dried and staked again. They 
are then " aged." 

To wet back for dyeing and finishing the skins are 
drawn through warm water and then drummed in water 
at 95 F. for 15 minutes to wet evenly and thoroughly. 
This liquor, which contains much of the tawing material, 
is run off and replaced by the dye solution, e.g. fustic or 
turmeric, with which the goods are drummed for half an 
hour. Iron, chrome or copper salts may be used for 
saddening. After this " bottom " colour is obtained, a 
coal tar colour is added for " topping " and the drumming 
continued until the required shade is obtained. The excess 
liquor is now run off, and the materials lost in soaking are 
replaced by drumming further with egg yolk and salt for 
15 minutes. This is known as " re-egging." Blacks are 



ALUM TANNAGES 177 

obtained with logwood and iron. After re-egging, the 
skins are dried out and staked. They are " seasoned " 
with a weak emulsion of soap and oil, dried, oiled lightly 
with linseed oil, ironed, re-oiled and finally brushed. Whites 
are undyed, and 10 lbs. French chalk per 100 skins is used 
in re-egging. 

" Calf kid " is a once popular but now obsolete upper 
leather made by tawing calfskins. The skins were well 
plumped in limes, delimed by washing and drenching, 
tawed much as for glove kid, split, dried out rapidly, staked 
and aged. They were finished dull and black with soap 
and wax. 

The various white leathers used for belts, laces, whip 
lashes, aprons, covers for stoppered bottles, etc., are very 
usually made with an alum tannage. Alum, salt and flour 
only are used. Whitening is also mixed in and acts as 
neutralizing agent as well as pigment dye. 

Wool rugs are manufactured from suitable sheepskins 
by an alum tannage. They are first well cleaned, using 
soap on wool and flesh. They are next degreased by 
painting with fuller's earth paste and drying. They are 
tawed by painting the flesh with a strong solution of alum 
and salt, or even by rubbing on the solid salts. They are 
dried out, aged and sorted for suitable colours. The 
dyeing is rather difficult, as many artificial dyestufls are 
of no use. It is usual to bleach the skins first in a weak 
solution of bleaching powder, and afterwards to dye with 
infusions of the dyewoods, e.g. logwood, fustic, sandal- 
wood, terra japonica, quercitron bark, turmeric, indigo, 
etc. Vat dyeing is usual. After dyeing, retanning with 
alum and salt is necessary, on account of the loss of these 
in bleaching and dyeing. Rugs are usually finished black, 
white, grey, brown, walnut, crimson, blue or green. 



REFERENCES. 

Procter, "Principles of Leather Manufacture," pp. 184, 236. 
Bennett, "Manufacture of Leather," pp. 239, 371. 

E. 12 



Section II.— FAT TANNAGES 

For the manufacture of a permanent leather the essential 
requirements are that the fibres of the hide or skins gel 
should be dried in a separate condition, and that they should 
be coated by some waterproof or insoluble material. Many 
substances fulfil the first but not the second of these con- 
ditions. For example, the dehydration only may be accom- 
plished more or less by salt (as in curing hides), still better 
by salt if a little mineral acid be used (as in pickling), and 
by other salts such as potassium carbonate and ammonium 
sulphate, and dehydrating agents such as alcohol. Such 
" temporary leathers," however, are not water-resisting, as 
the second requirement has not been fulfilled, viz. the 
coating of the fibres with some more or less waterproof 
material. Thus if pelts dehydrated with alcohol be treated 
with an alcoholic solution of stearic acid, the second con- 
dition is fulfilled and a permanent leather is obtained. 

Now, many tanning agents accomplish these two require- 
ments only imperfectly. As we have noted in the preceding 
section, the alum-tanned leathers are not very water resist- 
ing, and much of the tannage will wash out. Leathers 
made by the vegetable tannages usually contain some excess 
of vegetable tanning matters which are soluble in and 
removed by water, though much tannin can no longer be 
thus removed, owing to the mutual precipitation of the 
oppositely charged tannin sol and hide gel. The necessity 
for fulfilling the second requirement mentioned is one reason 
for the practice of following these tannages by applications 
of oil, fat or of both. In this way the isolated fibres are 
not only dried separately, but are coated with a typical 
water-resisting material. 

In the fat tannages an attempt is made to fulfil this 
second requirement without the use of any specific " tan- 
ning agent " for producing the first requirements ; i.e. an 



FAT TANNAGES 179 

attempt is made to dry the fibres separately in an " un- 
tanned " condition, and to coat them simultaneously with 
fat so that a permanent leather is obtained. It is only 
possible to do this, if the pelt is constantly during drying 
subjected to mechanical working, e.g. by twisting, folding, 
bending, drumming, staking, etc. The resulting leather is 
often called " rawhide leather," and presents a real advantage 
over other leathers in its great tensile strength. Where 
toughness is an essential quality, there is much to be said 
for the fat tannages. It is also possible, of course, to effect 
compromises between ordinary tannages and the straight 
fat tannages ; thus picking band butts, which must be 
tough, are often very lightly tanned with oak bark or 
chrome, and then given what is practically a heavy fat 
tannage. In the most typical of fat tannages, moreover, 
it is often common to " colour " the goods by a brief immer- 
sion in a weak vegetable tan liquor. Further, the employ- 
ment of fats in the currying of dressing leather is in effect 
a fat tannage superimposed upon the vegetable tannage. 
(See Combination Tannages, Section VI.) 

The fat tannage is undoubtedly one of the earliest 
methods for making leather. Prehistoric man discovered 
that the skins of animals killed in hunting could, by alter- 
nately rubbing with fats and then drying slightly, be 
eventually converted into a useful leather, whereas without 
the fat it was stiff and horny. Even yet similar methods 
are in use, thongs of raw hide being continually twisted during 
drying, with intermittent application of fats. 

In the modern fat tannages drums are used to give the 
necessary mechanical working to the goods. The raw hide 
leather produced in the U.S.A. is made by drumming the 
nearly delimed goods with tallow and neatsfoot oil. In this 
country the fat tannages have been typified by the " Crown " 
and " Helvetia " leathers. The hides are thoroughly limed 
in mellow limes, and after the beam work are delimed by 
drenching, scudded, and sometimes fleshed again, and then 
coloured off in tan liquor. After partial drying, they are 
drummed warm for some hours to ensure isolation of he 



i8o ANIMAL PROTEINS 

fibres. After further drying they are coated with the tanning 
paste, which consists essentially of soft fats and flour to 
produce partial emulsification. Equal parts of soft fats 
and of flour may be used, to which may be added smaller 
proportions of degas, cod oil, mutton tallow, salt, together 
with about 25 per cent, water. The goods are coated with 
this mixture, drummed, and dried further, and this routine 
repeated as often as necessary to fill the interstices thoroughly 
with fat. The temperature in the drum may reach 95 F. 
In finishing an attempt is made to stuff further with grease. 
The goods are thoroughly set out, dried a little, and coated 
again, flesh and grain, with a mixture of tallow, cod oil, 
glycerine and degras, and dried further. The excess grease 
is slicked off and the goods again set out and grained. They 
are then dried out. 



REFERENCES. 

Bennett, " Manufacture of Leather," pp. 245, 246 and 376. 
Procter, " Principles of Leather Manufacture," p. 378. 



Section III.— OIL TANNAGES 

There are very obvious analogies between the fat tannages 
discussed in Section II. and the oil tannages now to be dealt 
with, but there is nevertheless a distinct departure in principle 
involved. In the oil tannages the mechanical treatment is 
generally more vigorous, and the " drying " process is 
conducted at a much higher temperature, with the result 
that there is a vigorous oxidation of the oil. This results 
in the formation of insoluble oxidation products which coat 
the fibre and play an essential part in the production of a 
permanent leather. Pungent vapours are evolved in the 
drying operations, amongst which is acrolein and probably 
also other aldehydes, and it is thought by Procter that these 
aldehydes also are essential tanning agents and typical of 
the process (cf. Section IV.). Fahrion considers that the 
tanning action is due solely to unsaturated fatty acids with 
more than one double linkage. Garelli and Apostolo, 
however, believe that the tannage is due to a coating of 
fatty acid whether saturated or not. These observers made 
leather with stearic and palmatic acids in colloidal aqueous 
solution. 

The manufacture of chamois leather from the flesh 
splits of sheepskins comprises the largest and most typical 
branch of the oil tannages. The sheep pelts are split in 
the limed state, and the fleshes are given another sharp 
Hming which may last up to a fortnight. They are next 
" frized," i.e. scraped over the beam with a sharp two- 
handled knife, to remove roughness and loose fat. The 
goods are next thoroughly washed in running water and 
drenched. A paddle drench is often preferred, and if not 
used the handling should be frequent. Paddling drenching 
reduces the time required from about 16 hours to about 



182 ANIMAL PROTEINS 

6 hours. An hour or more in a hydraulic press removes 
superfluous liquor and some more grease. The fleshes are 
separated, cooled and then stocked for 30 minutes to equalize 
the moisture in them. After removing from the stocks 
they are sprinkled on both sides with cod oil and thrown 
back into the stocks for a few hours. They are then dried 
cold for a day or two. The stocks used are similar to those 
once popular for softening dried hides during soaking, and 
consist of two heavy hammers which fall alternately upon 
the goods which are contained in a curved box below. The 
result is a mechanical kneading action. The fleshes are 
again sprinkled with cod oil, restocked for a few hours and 
dried again, this time at ioo° F. They are then repeatedly 
sprinkled, stocked and dried, the last operation being 
conducted always at an increasing temperature until finally 
the final " heater " is even up to 160 F. As the operation 
proceeds it is advantageous to hang the splits also nearer 
one another, and in the final " heater " they are quite close. 
The next stage is to pack the goods quickly into suitable 
boxes and allow them to " heat," i.e. to oxidize further. This 
is a rather critical stage in the process, and to prevent over- 
heating (" burns ") it is often necessary to open out and 
repack into another box, with possibly some little intermediate 
cooling. They are turned over thus repeatedly until the 
oxidation is complete, and then spread out to cool. 

The fleshes are now a dark brown colour, and are next 
treated to remove excess of oxidized oil products. The 
goods are dipped through water at no F. and then subjected 
to hydraulic pressure. The grease and water which exude 
are allowed to separate by settling, and the thick yellow oil 
so obtained, known as " degras," forms a valuable material 
for leather dressing, as it more readily emulsifies with water 
than many oils, and impart this quality to other greases 
mixed with it. A further quantity of a similar oil is obtained 
by paddling the goods with a weak soda solution. The liquor 
obtained is treated with sulphuric acid to neutralize the 
alkali, and the grease recovered is known as " sod oil." The 
fleshes are now well washed with hot water (140 F.), fat 



OIL TANNAGES 183 

liquored with cod oil and soft soap, machine sammed, either 
by a wringer or a centrifuge, and then dried out. 

Much chamois leather is also made in France by closely 
similar methods. The skins are usually oiled on tables and 
folded up before stocking. Other marine oils (seal, whale, 
etc.) replace cod oil. Generally speaking the oxidation is 
more moderate, and the grease from the hydraulic press 
(moellon) is mixed with other fish oils to form commercial 
degras. An inferior quality of degras is obtained by subse- 
quent treatment with soda. 

The crust chamois obtained as above has only to be 
thoroughly staked to soften, " grounded " and " fluffed " 
to raise the nap, and then trimmed, and the ordinary wash- 
leather is obtained. 

If intended for glove leathers superior skins are selected. 
These are fluffed carefully upon emery wheels, using first 
a coarse surface and eventual^ a fine surface so that a fine 
velvet effect is attained. The skins are next bleached. 

In the " sun bleach " or " grass bleach " the goods are 
soaked in a i| per cent, soft soap solution and exposed to 
sunlight after being wrung. They are bleached in about 
3 days in summer, but nearly a fortnight may be necessary 
in winter. 

In the permanganate bleach, which is less tedious, the 
skins are first degreased by soaking in a warm f per cent, 
solution of soda crystals and then drumming for 30 minutes 
in water at 95 F. They are then paddled in a § per cent, 
solution of commercial permanganate for an hour at the same 
temperature, rinsed through water, and the brown manganese 
dioxide is then removed by paddling or drumming the goods 
in a 3 per cent, solution of sodium bisulphite to which 
hydrochloric acid is added as required. The goods are 
well washed in warm water, and are then " tucked," i.e. 
placed in a vat of boiling water containing a little soft soap, 
just for a few seconds. The goods shrink and curl up, and 
they are then dried out at i20°-i40° F. to fix the tuck. 
They are then staked, fluffed, and dyed. 

In dyeing with coal tar colours the alizarin colours may 



184 ANIMAL PROTEINS 

be used after mordanting with chrome alum. Direct dyes, 
natural dyestuffs and pigment dyes are also used. The goods 
are struck out after dyeing, lightly fat liquored with com- 
mercial egg yolk, dried out at no° to 120 F., staked and 
fluffed on the face side. 

Buff leather is a similar leather made from hides. They 
are limed mellow for a fortnight, unhaired, fleshed, and then 
limed again for another week in sharp limes. The grain is 
then split off, and the goods rinsed and scudded, slightly 
delimed and hung up to dry. They are then treated in 
much the same wa}^ as fleshes for chamois, but lime is often 
added to the cod oil used in stocking. 

Buck leather is a similar product obtained from deerskins, 
but much mock buck is made from cheaper raw material. 



REFERENCES. 

Bennett, " Manufacture of Leather," pp. 247-250 and 376-379. 
Procter, " Principles of Leather Manufacture," p. 378. 



Section IV.— FORMALDEHYDE TANNAGE 

The use of formalin for hardening gelatin has long been 
known, but it was left for Payne and Pullman to devise a 
commercial process for tanning pelt into leather by means 
of formaldehyde (H.CHO) solutions. Their process, which 
was patented, specified the use of alkalies in conjunction 
with formaldehyde or other aldehydes. The function of 
the alkalies is not very obvious, for it has been shown that 
formaldehyde will tan also in neutral and in acid solution. 
The precise action of the aldehydes is also as yet somewhat 
obscure, but it is noteworthy that very small proportions 
of formalin will give a complete tannage. It is probable 
that the action of formaldehyde is not perfectly analogous 
with that of its homologues, for it is a most reactive substance, 
and will certainly with proteids undergo reactions which 
are not analogous to those with other aldehydes. The 
leather obtained by tanning with formalin is quite white 
and resembles buff leather, but has advantages over the 
latter in that no bleaching is necessary. 

According to the patent specifications the pelt should be 
drummed in water and the tanning liquor— a solution of 
formalin and sodium carbonate— added gradually at 15- 
minute intervals. Up to 6 hours for light skins, and up to 
48 hours for heavy hides, are required for complete tannage. 
The temperature is raised during the process from ioo° to 
118 F. The tanning liquor may be made from 16 lbs. of 
commercial formalin (36 per cent, formaldehyde) and 32 lbs. 
soda (80 per cent. Na 2 C0 3 ) in 10-15 gallons of water. This 
should be added, one gallon at a time, to 4 cwt. pelt in 100- 
120 gallons of water. After tannage is complete the goods 
should be paddled with a \\ per cent, solution of ammonium 
sulphate to remove the soda, and " nourished ". in a solution 



186 ANIMAL PROTEINS 

of soft soap and salt, about z\ per cent, of each on the weight 
of pelt. The goods are then dried out, and may be finished 
like chamois, buff, and buck leathers (Section III.). 



REFERENCES. 

Payne and Pullman, English Patent 1898, 2872. 
Bennett, " Manufacture of Leather," pp. 250 and 379. 



Section V.— SYNTHETIC TANNING 
MATERIALS 

In spite of much valuable work on the constitution of the 
vegetable tannins and the compounds usually associated 
with them, such as that of E. Fischer, K. Freudenberg and 
their collaborators on gallo- tannic acid, and that of A. G. 
Perkin on ellagic acid and catechin, we are still in the dark 
with respect to the constitution of the tannins which are of 
commercial importance, and any synthetic production of 
these materials is thus out of the question as yet. Attempts, 
however, have been made to produce artificially substances 
which possess similar properties to the tannins and which 
may be used for converting pelt into leather. Into this 
category fall some of the earlier attempts to synthesize 
gallo-tannic acid by heating gallic acid with condensing 
reagents. 

The first commercial success in this direction was attained 
byStiasny, who produced condensation products of the phenol- 
sulphonic acids, to which products he gave the general name 
of "syntans" (synthetic tannins). The Badische Co. 
placed one of these products on the market as " Neradol D," 
and later took out subsidiary patents for the manufacture 
of similar products by slightly differing methods of pro- 
ductions. Since the outbreak of the European War such 
patent rights have been suspended, and several British 
firms have been manufacturing synthetic tanning materials 
by similar methods, but doubtless with developments and 
improvements of their own discovery. These products (e.g. 
Cresyntan, Maxyntan, Paradol, Syntan, etc.) are now in 
use in many factories, and assist rather than substitute the 
vegetable tannins in producing leather of the desired colour 
and quality. 

These synthetic tanning materials resemble the vegetable 
tannins in the following respects. They are organic acids 



188 ANIMAL PROTEINS 

containing phenolic groups. They are semi-colloidal, passing 
slowly through semipermeable membranes. They precipitate 
gelatin, basic dyestuffs and lead acetate, give a violet-blue 
colour with ferric salts, and convert hide into an undoubted 
leather. They differ from the vegetable tannins in that 
they contain sulphur and sulphonic acid groups, but they 
agree in that both are aromatic derivatives. In each case 
the tanning effect is diminished by alkalies, but the synthetic 
materials are the more sensitive. 

Methods of Manufacture. — There are, broadly speaking, 
three types of method by which these condensation products 
are produced, viz., condensation by formaldehyde, condensa- 
tion by phosphorus trichloride or similar reagents, and 
condensation by heat alone. Illustrative methods will now 
be given. 

Condensation by formaldehyde was the first method used. 
The procedure is given by the Austrian patent 58,405. A 
phenol, e.g. crude cresylic acid, is heated with the equivalent 
amount of sulphuric acid for a few hours to ioo°-2io° C, 
cooled, and formaldehyde added slowly whilst cooling and 
stirring, in the proportion of one molecule of formaldehyde 
to 2 molecules of phenol. The free mineral acid is neutralized, 
and the resulting product is the syntan " Neradol." By 
this procedure only water-soluble products are obtained, 
but an alternative process is to heat the phenols in slightly 
acid solution, and then to render soluble the resinous products 
obtained by treating with sulphuric acid. The proportion 
of formaldehyde to phenol used led Steasny to conclude that 
the resulting products were diphenyl-methane derivatives 
which polymerize to form molecules of considerable size. 
The formaldehyde supplies the " carbon bridge." This 
view was criticized by A. G. Green as too simple, and he 
suggested the alternative theory that polymerization does 
not take place at all, but that more advanced or higher 
condensation products are formed ; he thought that o- 
hydroxy-benzyl alcohols were first produced, that these 
condensed with another molecule, and afterwards the process 
was repeated. The result was a "colourless dyestuff," 



SYNTHETIC TANNING MATERIALS 189 

This view receives some support from the other types of 
method of manufacture. 

With the use of other condensing reagents the procedure 
may be as in the process of the B.A.S.F. (Fr. pat. 451,875-6), 
thus: 225 parts of o-cresol-sulphonic acid are heated to 
6o° C. for 4 hours with 262*5 parts of phosphorus oxychloride. 
The excess of oxychloride is removed by distillation under 
reduced pressure and the residue washed with dilute hydro- 
chloric acid. 

Condensation by heat alone is illustrated by the method 
given in the same patents, thus : phenol-p-sulphonic acid 
is heated to 130 C. for 24 hours under a pressure of 20 mm. 
or in a current of dry air at atmospheric pressure. The 
product may be used direct or may be purified by dissolving 
in water, neutralizing with caustic soda, filtering and evapo- 
rating to dryness. A white powder is obtained which tans 
when its solution is acidified. An alternative is to mix 
phenol with sulphuric acid and heat the mixture to 140 ° C. 
for 72 hours under 20 mm. pressure and purify as before. 

Methods of Use.— The synthetic tanning materials may 
be put to many uses. When well manufactured they make 
practically a white leather, and this fact makes a valuable 
opening for their use in connection with light leather tan- 
nages and the dressing of rugs. It is also claimed that they 
improve the colour usually obtained in the ordinary vege- 
table tannages. If used in the suspenders to the extent of 
5-10 per cent, they are said to brighten the colour through- 
out the tannage. If used in bleaching and finishing they 
are said to lighten the colour of the finished leather. About 
5 per cent, on the weight of the goods may be added to the 
bleach or vat liquors ; they may be also mixed with sumac 
during finishing, and in effect act as a sumac substitute ; 
solutions are also brushed over the grain before oiling, with 
a view to obtaining good colour. It is also claimed that 
their use prevents vegetable-tanned leather from becoming 
red under the action of sunlight. The syntans are also 
used to lighten the colour of chrome leather, even of chrome 
sole leather after it has been dipped. 



190 ANIMAL PROTEINS 

It is claimed also that syntans produce a tough leather, 
and if used for heavy leather in the early stages they give a 
tough grain and assist in avoiding a cracky grain. On this 
account they are also recommended for re-tanning E.I • 
tanned kips. When used in heavy leather suspenders they 
are said to get rid of lime blast (CaC0 3 ) and to quicken the 
tannage, i.e. to enable the same weight to be obtained in 
less time. Procter suggests that a tannage of commercial 
value might be obtained by blending them with wood pulp 
extract. 

If used alone for tanning a series of pits containing liquors 
of 4 to 37 Bkr. may be used, but drum tannages may be 
given using liquors of i4°-29° Bkr., the goods being tanned 
in 6-8 hours. About 30 per cent, of syntans are said to be 
necessary for complete tannage. 



REFERENCES. 

E. Stiasny, " A New Synthetic Tannin," Collegium, 1913, 142-145. 
(See also J. S.C.I. , Abs. 1913, 500.) 

E. Stiasny, " Syntans — New Artificial Tanning Materials," J.S.C.I., 

1913. 775- 

Patents : — Austrian 58,405. 

German 262,558, Sept. 12, 191 1. 

French 451,875, Dec. 13, 1912; 451,876, Dec. 13, 1912; 
45 r > 8 77. Dec. 13, 1912. 



Section VI.— COMBINATION TANNAGES 

The formation of leather being due to the adsorption of 
colloidogenic substances at the interface of the tanning 
liquor and the hide gel, there is the obvious possibility that 
several such substances may be used simultaneously, and 
that the resulting leather may be due to the combined effect 
of these substances. Indeed, the average vegetable tannage 
consists of such a combination tannage, each tanning 
material contributing its own individual tannin and 
characteristic astringent non-tannins. There is evidently 
also the possibility that the different types of tannage 
discussed above might be used either simultaneously or 
successively, and that a leather might be obtained which 
combines to some extent the qualities of each of the types 
in combination. It is such a case that is generally called 
a " combination tannage." There are many conceivable 
combinations, and in this section will be chiefly discussed 
a few which have demonstrated some commercial possi- 
bilities. Some of these have already received notice in the 
preceding sections. The manufacture of curried dressing 
leathers is a combination of vegetable and fat tannages. 
The manufacture of waterproof chrome uppers illustrates 
a combination of chrome and fat tannages. The use of 
" syntans " in conjunction with vegetable tanning materials 
is also a combination tannage. The case of chamois leather 
is possibly a combination of aldehyde tannage with fatty 
acid tannage. Two-bath chrome leather is a combination 
of chrome, sulphur and fat tannage. Formaldehyde and 
vegetable tannage is also a known possibility. It is clear 
that there are possibilities of endless complexity, and that 
what normally may appear as a simple tannage is in reality 
a very complex combination tannage. From this standpoint 
one might instructively consider the successive adsorptions 
involved in a goatskin tanned first with syntans, then with 



192 ANIMAL PROTEINS 

oak bark, " retanned " in sumac, mordanted with chrome, 
dyed with coal-tar dyestuffs and finally oiled with linseed 
oil. It will be easily seen that in a very strict sense nearly 
all tannages are combinations. 

Usually, however, the term " combination tannage " is 
confined to those cases where the main tanning agents not 
only differ in type, but where none are in predominant 
quantity. A typical case is that of " semichrome leather," 
in which a vegetable tannage is succeeded by a chrome 
tannage. E.I. tanned sheep and goat skins are rather 
heavily " stripped " of their vegetable tannage and heavy 
oiling, by drumming with warm soda solutions, and after 
washing with water are chromed with the one-bath process ; 
they are neutralized, dyed, fat liquored and finished for glace 
upper leather. 

In a precisely similar way kips and split hides which have 
received vegetable tannage are stripped and retanned in 
chrome and finished as for box calf, of which they are a good 
imitation. Such vegetable-chrome combination tannages 
possess many of the properties of chrome leather. 

To chrome the pelt first and afterwards to subject it to 
vegetable tannage is also an obvious possibility, but has not 
yet been made a commercial success in this country, but has 
been increasingly used in the U.S.A. during the War. 

Another typical case of combination tannage is the 
dongola leather produced by the use of gambier and of 
alum and salt. This is a vegetable-alum combination, and 
yields a good quality leather for light uppers, gloves, etc. 
Goatskins for " glazed dongola " are paddled tanned in 
gambier liquors, and alum and salt are subsequently added. 
They are tanned in 24 hours, well washed, and are fat 
liquored without ageing. The E.I. tanned skins may also 
be stripped with soda, and retanned in alum and salt, using 
flour also if desired. Dull dongola are first tawed and then 
retanned in gambier liquor. " Suede " and " velvet calf " 
are also tawed and retanned with gambier. 

Yet another case of combination tannage is that of sheep- 
skins for glace uppers, which are first tawed thoroughly 



COMBINATION TANNAGES 193 

with alum, salt and flour and dried out for sorting, and are 
then retanned in chrome by the one-bath process, and 
finished as usual. Closely related to this is the method of 
" pickling " in alum and salt and then chrome tanning. 

Another case is the combined one-bath, two-bath method 
of chrome tanning. The goods are chromed by a one-bath 
liquor containing dichromate (say 2 per cent.), and then pass 
into a reducing bath. There is not much advantage in such 
procedure, however. 

From a strictly commercial point of view the " dongola " 
and " semichrome " leathers have proved the most successful 
combination tannages, but there seem to be possibilities in 
combinations of the vegetable tannins with synthetic tanning 
materials. 

Many other substances are known to tan, e.g. iron salts, 
cerium salts, sulphur, quinones, fatty acids, the halogens, 
etc., etc. ; hence there is always the possibility that new 
useful combination tannages may be discovered. 



REFERENCES. 

Bennett, " Manufacture of Leather," pp. 243, 374-5. 
Procter, " Principles of Leather Manufacture," p. 236. 



13 



Section VII.— THE EVOLUTION OF THE 
LEATHER INDUSTRY 

The leather trades are amongst the oldest of all industries, 
but their evolution has been much more rapid during the 
last two or three decades than at any other period of their 
history. The European War, moreover, has caused the 
commencement of another period of rapid development, 
and it is the aim of this section to point out some of the 
principal lines of change which have already become apparent. 

Many of these lines of evolution in the methods of 
manufacture have been previously discussed in their appro- 
priate sections. They may all be summarized as attempts 
at more economical production. Prominent amongst them 
is the persistent effort to attain quicker processes. During 
the last twenty-five years the time necessary to produce the 
heavy leathers has been reduced from 12 months to as 
many weeks. The tendency is to reduce the time further 
still, but this is of course increasingly difficult to accomplish. 
On the other hand, it is more urgent to strive in this direction 
than ever, because a needless week involves more capital 
lying idle than ever before. Moreover, as most leather 
factories are now large works, a saving even of 24 hours 
has become a serious item in economic production. Hence 
in liming, bating, tanning, drying and in warehousing there 
are increased efforts to make a quicker turnover. 

A good illustration of this " speeding up " in modern 
tanneries is the adoption by all large factories of much more 
rapid methods of extracting tannin. On the old press-leach 
system liquors may be percolating through the material for 
possibly a fortnight. The extract manufacturer reduces 
this operation to about two days. Steam generated from 
the spent bark is used to heat the extracting vats, and to 
work a vacuum pan or evaporator whereby more water can 
be used and a more complete as well as a more rapid 



THE EVOLUTION OF THE LEATHER INDUSTRY 195 

extraction obtained. The evaporator also makes easy the 
preparation of the strong liquors used in modern tanning. 

Hand-in-hand with quicker production and manipulation 
are the attempts to obtain a larger turnover. It is realized 
that the big business attains cheap production. Even before 
the war the smaller factories were disappearing. A small 
tannery must now either extend or close down. This has 
been better realized in the heavy than in the light leather 
trades. In the sole leather tanneries very often many 
thousand hides per week are put into work, but in the glace 
kid factories there is nothing yet to correspond to the 
output of American glace factories, which sometimes reaches 
three or four thousand dozen a day. 

Another very prominent feature of factory evolution 
is the increased use of labour-saving machinery. This 
practice has been in operation for a considerable time, but 
with marked acceleration during the last few years owing 
to the labour shortage occasioned by military service. This 
development of machine work has largely dispensed with 
that labour which involved any skill or training. The 
journeyman currier is now practically extinct. In the 
beam house, too, fleshing, unhairing and scudding are 
rapidly becoming machine instead of hand operations. 
Many devices are now being adopted also which reduce the 
quantity of unskilled labour needed. Instead of " handling " 
the goods from pit to pit, modern tanneries aim at moving 
the liquors. Thus in the " Forsare " and " Tilston " systems 
of liming, hides are placed in a pit and lie undisturbed until 
ready for depilation, the soak liquors and lime liquors being 
supplied and run off just as required, whilst these liquors 
are agitated as often as desired by means of a current of 
compressed air. This agitation replaces the " handling " 
up and down once practised. In the tanyard proper the 
same tendency is at work, " rockers " are increasingly 
preferred to " handlers." and an inversion of the press leach 
system permits the exhaustion of tan liquors by a gravity 
flow, and so avoids the handling forward from pit to pit. 
There is also a tendency to install lifts, overhead runways, 
trucks on lines, motor lorries, etc., to replace carrying, 



196 ANIMAL PROTEINS 

harrowing, carting, etc., and so to arrange the tannery that 
the minimum transport is needed. 

All these lines of evolution involve more intensive pro- 
duction, and necessitate much more careful supervision. 
It is not surprising, therefore, that the industry now feels 
that scientific oversight and administration are essential. 
A dozen years ago the trade chemists were largely un- 
qualified men, whose work lay solely in the laboratory, 
and consisted mainly in the analysis of materials bought. 
To-day all large tanneries have qualified chemists, and it is 
realized that they are the practical tanners. Their function 
is so to control the manufacturing processes that all waste 
is avoided, and so to correlate and co-ordinate the manu- 
facturing results with the analytical and experimental records 
of the laboratory, that constant improvements are made in the 
methods of production. The extended use of machinery, and 
the necessity for economy in coal and power, give the engineer 
also very large scope for useful work. Modern business con- 
ditions, moreover, have made necessary more skilful clerical 
work and accountancy in the large offices of a modern tannery. 
In the creation of cordial relationships between capital 
and labour in the leather trades, there has been un- 
fortunately little progress. The leather trade is not a 
sweated industry. Its workers have always enjoyed reason- 
able hours of work. In most factories an approximate 
48-hour working week (involving no night work) has long 
been in operation. The industry, however, is not one in 
which high wages obtain. The average tannery worker 
receives a wage which is never much above the level of 
subsistence. This is mostly due to the fact that he is 
usually a quite unskilled labourer, and is therefore on the 
bottom rung of the labour ladder. In addition to this the 
work itself is often distressingly monotonous, and makes 
little demand upon the intelligence of the worker. The 
trade consequently offers little attraction to the intelligent 
labourer. The old system of apprenticeship is now quite 
obsolete, partly owing to the rapidity of the changes in the 
methods of manufacture, partly to the specialization of 
labour which results from the development of large factories, 



THE EVOLUTION OF THE LEATHER INDUSTRY 197 

and partly also, because to understand modern tanning 
involves a better general education than most workmen 
receive. It is indeed frequently difficult to find competent 
under-foremen for trie different departments of the modern 
leather factory. Until recently leather workers have been 
either unorganized or badly organized, and their views and 
complaints have been confused and sporadic, but during 
the war period there has been a very rapid extension of 
trade union movements, and consequently a more articulate 
expression of the demands for " democratization " as well 
as " a greater share in the fruits " of the industry. In the 
leather trades, however, the gulf between the unskilled 
labourers and the wealthy employers is perhaps unusually 
wide, and there is little disposition on the part of capital to 
recognize the equity of either of the above demands of 
labour. Generally speaking, the leather trade firms are not 
public but private companies. There is absolutely no trace 
of "co-partnership" or "profit-sharing" schemes, or of 
co-operative production. There is little recognition that the 
trades' prosperity should be shared in any way by the work- 
people, and still less recognition of any right to a voice in in- 
dustrial conditions. This condition of affairs has an ominous 
reaction upon the attitude of labour, which believes that it is 
producing great wealth but not obtaining much more than 
subsistence. It is not the function of this volume to pronounce 
a verdict upon the wages question or upon the democratiza- 
tion of the leather trades, but one may be permitted earnestly 
to hope that if such be the future lines of development, there 
will be also, as an absolutely essential part of any such 
schemes, a much higher standard of education amongst the 
workers, for this is the only satisfactory guarantee that the 
voice of labour in council will have any practical value, or 
that higher wages will be at all wisely used by the recipients. 
In his instructive and valuable volume on " The Evolu- 
tion of Industry," Prof. MacGregor points out that modern 
industry has evolved three outstanding types, viz. the 
Co-operative Movement, the Trusts, and the methods of 
Public Trading. He also suggests that these types tend to 
blend. In the leather industry co-operative and municipal 



198 ANIMAL PROTEINS 

production are unheard of, but the industry has certainly 
developed along the lines of the large trusts. Large busi- 
nesses have replaced small, and later still have formed local 
federations, which in turn have combined to form the 
" United Tanners' Federation." War conditions have 
certainly stimulated evolution towards the trust type. The 
United Tanners' Federation has become possessed of powers 
which were not originally contemplated, such as the purchase 
and distribution to its members of hides, bark, extract, sul- 
phide .and other materials. How far some of these arrange- 
ments will be permanent is problematical, but one beneficial 
result is that the allied trades have certainly realized more 
thoroughly their unity of interests. This is shown by the much 
freer collaboration of the tanners, and by the encouragement 
now given to similar collaboration between their chemists. 
More evidence is found in the proposals for combined research. 
There is also considerable reason to believe that there 
is some movement in the direction of partial State control. 
There is little doubt that evolution along trust lines will 
make this less difficult and possibly more desirable. The 
country cannot afford the spectacle of a Leather Trust 
permanently at war with a Labourers' Union. The public 
has realized that the well-being of the leather industry is 
vital to the national safety. It has realized that the leather 
trades are great producers of national wealth, and that 
increased production with the development of the export 
trade will materially assist to restore the country's financial 
position. It has realized also its own right to protection 
from bad leather and from exorbitant prices. On all these 
grounds it is probable, though there may be some reaction 
from the present position, that the State, which has already 
got its fingers in the pie, will refuse to draw them out alto- 
gether. The Imperial aspect of the question affords some 
further justification for this attitude. The leather trades 
operate very largely upon imported material, and it is clearly 
desirable that there should be close co-operation between the 
home industry and the colonial supplies of material. Here 
too the war has also given a great stimulus in this direction. 
Indian myrabolans has long been a staple tanning material. 



THE EVOLUTION OF THE LEATHER INDUSTRY 199 

South African wattle bark has during the last few years 
replaced almost completely, and probably to a large extent 
permanently, Turkish valonia. There has also been great 
increase in the imports of Indian kips and of South African 
hides, and it is not at all an impossible proposition to main- 
tain a self-contained Imperial Leather Trade, should this be 
necessary. French chestnut extract, and quebracho extract, 
however, are much too valuable tanning materials to exclude 
for merely sentimental reasons. These instances indicate 
possible advantages in Imperial co-operation, but also show 
the need for caution in the elaboration of such schemes. 

Although a partial, and indeed increasing, measure of 
State Control is probable, there has been as yet no serious 
proposal to nationalize the leather industry. Such a pro- 
position, indeed, is hardly ripe even for discussion. Until 
the nationalization of transport and of mines is a proved 
success, and until the merely distributive undertakings of 
the municipalities {e.g. of coal and of milk and other foods) 
are past the experimental stage, any proposition to nationalize 
the leather trades seems premature. It is noteworthy, 
however, that in Queensland, Australia, the Government have 
the right to commence and to administer State Tanneries. 

Any progress in the direction either of democratization 
or of nationalization, has been certainly postponed by the 
sudden and unprecedented trade slump which commenced 
in the earlier part of 1920. This depression, in spite of heavy 
falls in the prices of raw materials, has made economic 
production a much more difficult problem. It has un- 
doubtedly given a further stimulus to evolution towards the 
trust type, and created a further tendency towards the closing 
of the smaller factories, and the employment of labour-saving 
devices. When the general fall in prices has made an appreci- 
able fall in the cost of living, some reduction in the leather- 
workers' wages, together with more efficient work, will also 
contribute to the solution of the difficulty. It is, chiefly to 
be desired, however, that the export trade should be restored. 
The realization of this hope depends largely upon the 
establishment of peace and prosperity abroad, and the con- 
sequent stabilization of the various foreign exchanges. 



Part V.— GELATINE AND GLUE 

Section L— PROPERTIES OF GELATINE 
AND GLUE 

Many of the chemical properties of gelatine, especially those 
which distinguish it from other proteins, have been described 
in the Introduction to this volume, and need no further 
comment. In this section its colloid nature and behaviour 
will chiefly be considered, for these points have greatest 
importance from the standpoint of industrial chemistry. 

It is hoped, moreover, that this section will be of interest 
not only to the chemist concerned in the manufacture of 
gelatine and glue, but that it will be of value also to those 
concerned in leather manufacture. The difference between 
the " collagen " which composes the hide fibre and the 
high-grade gelatines is so small that for many practical 
purposes it may be considered negligible. Thus the de- 
scription of the behaviour of a gelatine gel is very largely 
applicable to a hide gel also. 

Gelatine has been crystallized by von Weimarn by 
evaporating a dilute solution in aqueous alcohol whilst in a 
desiccator containing potassium carbonate, the temperature 
being maintained at 6o°-70° C. The carbonate takes up 
water only, and the concentration of the alcohol therefore 
slowly increases until the gelatine is no longer soluble. 
Gelatine is usually found and known in the colloid state, 
however, and its behaviour in this state only is of practical 
importance. 

The fundamental idea of modern colloid chemistry is 
that colloids are heterogeneous systems, usually two-phased, 
in which one phase is liquid and the other phase either 
liquid or solid. The latter phase, which is divided into smalj 



PROPERTIES OF GELATINE AND GLUE 201 

separate volumes, is known as the "disperse phase," whilst 
the other is the "continuous phase" or "dispersion 
medium." The " dispersity " is the degree to which the 
reduction of the dimensions of the disperse phase has been 
carried, and is best expressed numerically in terms 01 
" specific surface," i.e. surface area divided by volume, but 
it is also often expressed as the thickness or diameter of a 
film or particle. When the dispersity is not high, we have 
ordinary " suspensions " and " emulsions," which with 
increasing dispersity merge into the typical colloids. By 
analogy, colloids have been divided into " suspensoids " and 
" emulsoids," when the disperse phase is solid and liquid 
respectively. The classification, however, has not been 
found satisfactory, for some systems in which the disperse 
phase is undoubtedly liquid, exhibit characteristic properties 
of suspensoids, and vice versa. A more satisfactory division, 
therefore, is found in the presence or absence of affinity 
between the two phases, the systems being termed " lyophile " 
and " lyophobe " respectively. If water be the continuous 
phase the terms " hydrophile " and " hydrophobe " are 
often used. Broadly speaking, the lyophile colloids corre- 
spond to the emulsoids, and the lyophobe colloids to the 
suspensoids. Gelatine is a typical hydrophile colloid. 

Another fundamental idea of colloid chemistry is that the 
great extension of surface involved in a high dispersity 
causes the surface energy to be no longer a negligible fraction 
of the total energy of the system, and that the recent 
advances in knowledge respecting surface phenomena may 
be called in to assist in the explanation of the special pro- 
perties of the colloid state. Particles which exhibit the 
Brownian movement, about io~ 5 cm. diameter, down to the 
limit of microscopic visibility (10 -3 cm.) are termed microns. 
Particles less than this, but just visible in the ultra-micro- 
scope (5X10 -7 cm.) are termed submicrons. Particles still 
less, approximately 10 ~ 7 cm., have been shown to exist, and 
are termed amicrons. The dimensions of molecules such as 
may exist in true solutions are of the order of 10 ~ 8 cm. A 
colloid sol may contain particles of various sizes. Thus a 



202 ANIMAL PROTEINS 

gelatine sol (like other lyophile systems) contains chiefly 
amicrons, but submicrons are also observable. 

i. The Continuous Phase 

Owing to the contractile force of surface tension, it is 
concluded that the surface layer of a liquid is under very 
great pressure, much greater than the bulk of the liquid. 
Any extension of the surface of the liquid naturally causes 
a corresponding extension of the proportion of liquid which 
is thus compressed. If in a beaker of water there be placed 
a porous substance, such as animal charcoal, there is a great 
extension of the surface of the water, and a corresponding 
increase in the amount of compressed water. If instead 
there be substituted a large number of very small particles 
of a substance, a still further increase in the amount of com- 
pressed water is involved. As the specific surface of the 
substance inserted is increased, and its amount, the propor- 
tion of compressed and denser water increases also, until it 
is a practically appreciable percentage of the total volume. 
It is clear also that the extent of the zone of compression 
will be determined also by the nature of the substance with 
which the water is in contact at its surface, i.e. by the extent 
to which it is hydrophile, and this indeed may be the more 
important factor. 

Now in a gelatine sol we have the necessary conditions 
for a system in which the compressed water bears an un- 
usually large ratio to the total, owing to the enormous 
surface developed by the minute particles of the disperse 
phase (amicrons) and to the unusually wide zone of com- 
pression surrounding each particle caused by the strongly 
hydrophile nature of gelatine. It should be pointed out 
that these zones of compression do not involve any abrupt 
transition from the zone of non-compression, the layer 
nearest the particle is under the greatest pressure, and the 
concentric layers under less and less pressures, the actual 
compression being thus an inverse function of the distance 
from the particle. Now if there be a gradual increase in the 
concentration of the sol, the time will come when these 



PROPERTIES OF GELATINE AND GLUE 203 

zones of compression begin to come in contact, and the 
system will then show a considerably increased viscosity. 
With further increase in concentration the zones of com- 
pression will overlap throughout the system, and when the 
layers under considerable pressure are thus continuous, the 
whole system will acquire a rigidity much greater than water 
and approaching that of a solid body. This is a gelatine gel, 
or " j elly . ' ' With increasing concentration the j elly becomes 
increasingly rigid, and if it be eventually dried out under 
suitable conditions it forms what is practically solid body — 
gelatine — which, however, still contains from 12 to 18 per 
cent, of water. 

It will be clear that, in the case of gelatine jellies (e.g. of 
3-10 per cent, strength), an increase in temperature will 
cause an increase in the kinetic energy of the particles and 
effectively reduce the zones of compression. Indeed, they 
may be reduced to such an extent that they are no longer 
in contact, and the rigidity due to the continuous contact of 
the layers of great compression will then disappear ; as we 
say usually, the jelly melts. On cooling, the decreased 
kinetic energy of the water molecules results in the return of 
the state of compression, with rapidly increasing viscosity 
and eventual gelation ; as we say usually, the jelly sets. 
Neither of these changes takes place at a definite temperature 
(like a melting-point), and in "melting" (solation) or in 
" setting " (gelation) the temperature-viscosity curve is 
quite continuous. By various arbitrary devices, however, 
approximate melting and setting points may approximately 
be determined. The results also vary somewhat with the 
concentration of the gel or sol. Gels between 5 and 15 per 
cent, strong melt about 26°-30° C. and set at i8°-26° C. 

On this view, we must regard a gelatine gel as a continuous 
network of water under great compression, and in this net- 
work are zones of still greater compression, which surround 
the particles of the disperse phase — the gelatine itself, and 
zones of less compression which in a weak gel, at any rate, 
have a compression equal to or much the same as the normal 
state of compression in water. 

One consequence of this system is, that when a piece of 



204 ANIMAL PROTEINS 

gelatine swells, there is a considerable enlargement in the 
zones of compression ; in other words, some, at least, of the 
imbibed water is compressed. Now the compression of 
water means that work is done, and when gelatine swells, 
therefore, we expect — and actually find — that heat is 
liberated (57 cal. per g. gel). Hence also by the I^e Chatelier 
theorem, we expect — and find — that gelatine swells best in 
cold water. Further, the compression of water involves a 
decrease in volume, and we therefore expect — and actually 
find — that the volume of the swollen jelly is appreciably less 
than the volume of gelatine plus the volume of water imbibed. 

Another consequence of such a compressed system is 
that a gelatine jelly, even in water, will have a surface tension 
towards water just as the water itself has such a tension to 
the water vapour above the liquid. This interfacial tension 
of the jelly will of course have a contractile effect, and will 
tend to resist swelling and to limit it as far as it possibly can. 
This force, tending to contract the jelly and resist imbibition 
is therefore one of the main influences at work in the swelling 
of gelatine, and is one of the two principal factors which 
determine the extent of the maximum swelling when equili- 
brium is established. The force tending to resist swelling is, 
in the ultimate, just surface tension. Its actual magnitude 
depends, of course, mainly upon the extent of compression 
in the dispersion medium of the gel, and will be a resultant 
which is a function of this compression. The magnitude 
will thus vary with the average compression in the con- 
tinuous network of compressed water. It will be obvious 
that as the jelly swells the power of resisting the swelling 
will decrease, and the interfacial tension with the external 
water will tend to disappear. If the force tending to swell 
were great enough the swelling would continue until the 
zones of compression were no longer in contact and the gel 
would become sol. 

As suggested above, it is probable that the extent of the 
zones of compression is determined by another factor in 
addition to the great development of surface. That factor 
is connected if not identical with that power which makes 



PROPERTIES OF GELATINE AND GLUE 205 

the system lyophile, and is evidently connected also with the 
solubility of the disperse phase, and may indeed be electro- 
chemical forces tending to form a series of hydrates, or at 
least to cause an orientation or definite arrangements of the 
water molecules in the zone of compression. This idea 
receives some support from the hydrate theory of solution, 
and the zones of compression and orientation are the colloid 
analogue of the hydrates supposed to exist in solutions of 
electrolytes. The extension of such zones on cooling are 
then analogous with the series of hydrates formed, for 
instance, by manganese chloride with 2, 4, 6, 11, or 12 
molecules of water when crystallized at temperatures of 
20°, 15 , —21 , — 30 , and — 48 C. respectively, the idea 
being that the salts most hydrated in solution crystallize 
with most water. 

As the compression is the result of two factors, one of 
which depends upon the nature of the disperse phase, we 
expect — and find — in other lyophile systems a considerable 
variation in their power of gelation. Some indeed, though 
very viscous, e.g. egg albumin, never quite set like gelatine, 
and others (e.g. agar-agar) set to a stiff gel from a much 
weaker sol than gelatine. When the zones of compression 
are large, as in gelatine, the magnitude of the compressing 
force on the outermost part of the zone is relatively small, 
and it is not surprising that time is necessary for the victory 
of this force over the kinetic energy of the water molecules. 
Hence we find a 5 per cent, jelly sets readily on cooling, but 
its elasticity increases steadily for many hours after it has set. 
This phenomenon, known as hysteresis, we should expect — 
and find — to be much more marked in a case where the zone 
of compression is unusually large (e.g. an agar gel). We 
should also expect — and find — that hysteresis is more marked 
in a high-grade gelatine than in a low-grade gelatine where 
both eventually form gels of equal elasticity. We should 
expect too — and we find — that hysteresis is more prominent 
in weak gels than in strong. These points are of obvious 
importance in testing gelatine by its elasticity, e.g. the well- 
known " finger test." 



206 ANIMAL PROTEINS 

There are also other facts and considerations which have 
an important bearing upon the point under discussion. It 
is necessary ultimately to regard true solutions of electrolytes 
and other bodies as heterogeneous, though perhaps of a 
rather different order. From this point of view molecules 
and ions existing in an aqueous solution will present a surface 
and have associated zones of compression analogous with 
those suggested for the minute particles of gelatine. 

Now recent investigations have shown that the essential 
physical properties of water are affected by dissolved sub- 
stances in a definite manner and to a fixed extent, and that 
these substances exhibit a sequence in order of their effect. 
This sequence is also exhibited in the essential properties 
of water as solvent and as dispersion medium for colloid sols. 
The sequence is known as the " lyotrope series." Thus the 
numerical value of the compressibility of aqueous solutions 
is reduced below that of water by salts which, with the same 
kation, exhibit an effect in the following order : — 

C0 3 >S0 4 >Cl>Br>N0 3 >I 

This same order is observed, in the effect on the increased 
values for the surface tension, density and viscosity of these 
solutions. On the other hand, the kations have a similar 
sequence of effects, 

Mg < NH 4 < Li < K < Na < Rb < Cs 

which appears when salts of the same anion are chosen. It 
is not surprising to find that this lyotrope series exhibit an 
analogous influence on the chemical reactions of water, e.g. 
the hydrolysis of esters. In the hydrolysis by acids S0 4 
retards the action, the other anions and the kations accelerate 
it, in the lyotrope order. In the hydrolysis by bases the 
series is reversed. Similarly the lyotrope series exert the 
same order of effect upon the inversion of cane sugar and 
other reactions. 

This lyotrope influence has also been shown to exert 
considerable effect in the behaviour of lyophile sols. With 
the lyophobe sols the addition of foreign substances ap- 
parently affects the disperse phase only, but with the 



PROPERTIES OF GELATINE AND GLUE 207 

lyophile sols the effect on the continuous phase is also 
important, and may overshadow the other. Now, in 
gelatine and in hide gels and tanning sols we are dealing 
with lyophile systems, and there are many points of behaviour 
in which ly otrope influences become prominent. Similar effects 
are observed upon other lyophile sols (e.g. albumin, agar-agar, 
etc.) which differ widely in chemical nature. Thus the 
salting out of albumin (reversible precipitation) is influenced 
by sodium salts in lyotropic sequence as follows. The anions 
hinder precipitation; in order of precipitating power they are : 

citrate > tartrate > S0 4 > acetate > CI > N0 3 >C10 3 >I>CNS 

The sulphates illustrate the kation effect, which is indepen- 
dent and which favours precipitation : 

I4>K>Na>NH 4 >Mg 

If the experiments be carried out in faintly acid solution this 
order of effect is exactly reversed, iodide and thiocyanate 
having the greatest effect and citrates the least. The coagula- 
tion temperature of albumin and the coagulation by other 
organic substances are similarly influenced by the lyotrope 
series. 

Lyotrope influence also exerts a powerful effect on the 
behaviour of gelatine sols and gels. The gelation tempera- 
ture is influenced thus : — 

raised by S0 4 > citrate > tartrate > acetate 
lowered by CI < C10 3 < N0 3 < Br < I 
The kation effect (small) is Na > K > NH 4 > Mg 

Other lyotrope substances raise or lower the temperature 
thus : — 

glucose > glycerol — (H 2 0)— alcohol < urea 

The effect on gelation is also illustrated by the change of 
viscosity of the sol with time. The same lyotrope order is 
found. 

In the salting out or precipitating of gelatine with salts, 
the order of anions is lyotrope : 

SQ 4 > citrate > tartrate > acetate > CI 



208 ANIMAL PROTEINS 

Also the osmotic pressure of gelatine sols is markedly lowered 
by neutral electrolytes in lyotrope sequence : 

Cl>S0 4 >N0 3 >Br>I>CNS 

Similarly lyotrope influences are shown in the modulus 
of elasticity : substances which favour gelation increase 
elasticity, whilst substances which favour solation decrease 
elasticity. The order is again lyotrope. 

The permeability of the gel is affected by lyotrope in- 
fluences ; alcohol and glycerol reduce diffusion through 
gelatine (or agar) ; and urea, chloride and iodide increase it. 
(Similarly the diffusion of sols through " semipermeable " 
membranes is affected by lyotrope influence.) The lyotrope 
series also influence the optical activity of gelatine sols and 
the double refraction of strained gels. 

The swelling of gelatine (and other gels) is very strongly 
influenced by the lyotrope substances and merits more 
attention than it has received. Hence this lyotrope influence 
exerts a profound effect in the manufacture of gelatin, and 
perhaps even greater in the manufacture of leather. This 
is only to be expected. If a gel comprise a continuous 
network of compressed water, as suggested above, the 
presence of other substances in the gel which cause increases 
or decreases in the compression must modify accordingly 
the properties which depend upon this state of compression, 
such as the viscosity of the melted gel, the rate of gelation, 
the elasticity of the gel, and the rate and extent of its 
imbibition. This indeed we find to be the case. Now 
the substances which affect the compressibility, surface 
tension, etc., of water least, i.e. the substances producing 
little or no compression of water, are just those which 
reduce the compression of water in a gelatine jelly, and cause 
a decreased viscosity, elasticity, surface tension, etc., and 
which therefore naturally allow the gel to swell more than 
in pure water. Conversely, the substances which cause the 
greatest compression of water, the greatest increase in its 
surface tension and viscosity, are also the substances which 
increase the compression, viscosity, elasticity, and surface 



PROPERTIES OF GELATINE AND GLUE 209 

tension of gels, and which therefore hinder imbibition. The 
effect on swelling is as follows : — 

Sodium-snlphate > tartrate > citrate > acetate ; > alcohol > 
glucose > cane sugar ; (water) chlorides-potassium < sodium 
< ammonium ; < sodium-chlorate < nitrate < bromide < 
iodide < thiocyanate < urea. 

As the amount of compression will depend upon the 
amount of substance, we expect — and find — that the effect 
is usually additive, and that suitable mixtures of substances 
having an effect in the opposite sense will produce no change. 

The interpretation of lyotrope influence is of course 
somewhat speculative, but considered as a surface pheno- 
menon, the surface specific of the molecules and ions of the 
lyotrope substance must be one of the factors involved. 
One naturally also connects the effect with solubility and 
the tendency to form hydrates in solution, the zones of com- 
pression being zones of orientation and of electro-chemical 
attraction. The hydrate theory of solution again affords 
an instructive commentary. The fact that, broadly speaking, 
the polyvalent anions and the monovalent anions also group 
themselves together, suggests that electrical forces are at 
work, and the order of effect of monovalent anions almost 
suggests that what are called " residual valencies " are in 
operation. It is difficult to resist the conclusion that in the 
lyotrope influence, in the crystallizing of salts, and in the 
formation of a gel, we have zones of compression and orienta- 
tion which are manifestations of the same forces — surface 
and electrical ; the chief differences in the case of gelatine 
being that the zones are larger and that the electrical effect 
is perhaps of less definite magnitude. 

However these things may be, the fact of water com- 
pression determines the rigidity of the gel, and the changes in 
this compression of the continuous phase determine the 
surface tension resultant which hinders swelling, and which 
is one of the two main factors fixing both the rate at which 
gelatine swells in water, and the final volume attained by 
the gel. 

E. 14 



210 ANIMAL PROTEINS 

Before leaving this point, it is desirable to note the effect 
on the swelling of gelatine of the extremes of this lyotrope 
influence. Substances like iodides, thiocyanates and urea 
prevent a gelatine sol from setting to a gel at all, and a piece 
of gelatine in such solutions swells rapidly until it solates. 
On the other hand, sulphates, tartrates, etc., make a stiff er 
gel on account of the enhanced compression. Gelatine in 
such solutions may swell, but at a much slower rate than in 
water and with a decreased maximum extent. A gelatine 
gel may in such solutions not only fail to swell at all, but 
actually contract and in some cases, indeed, be practically 
dehydrated. If a gel be in a very concentrated solution of 
such a substance, it may be that the lyotrope compression 
in the external solution is greater than the compression in the 
dispersion medium of the gel ; in which case the surface 
tension effect is reversed, and the external solution tends to 
increase in volume and the gel to contract. Hence we find 
that the saturated solutions of such substances as ammonium 
sulphate and potassium carbonate will dehydrate a gel almost 
completely, and will also, by a similar action on pelt, make a 
kind of white leather. It is important to remember this 
contractile effect of strong solutions of salts, because it is 
very easy to confuse this effect with a similar result produced 
in another manner, viz., by a reduction of the force tending 
to swell. 

2. The Disperse Phase 

A very important feature of the colloid state is that the 
particles of the disperse phase appear to possess an electric 
charge, and if this charge be removed a colloid sol no longer 
remains such, but precipitates, floculates, coagulates, etc. 
As to the origin of this charge several theories have been 
advanced, but the most generally accepted is that it is a 
result of the adsorption of electrically charged ions by the 
particles of the disperse phase. The enormous specific 
surface possessed by this phase renders it particularly liable 
to such adsorption. This view harmonizes well also with 
the general behaviour, of colloid sols and gels, in endosmosis, 



PROPERTIES OF GELATINE AND GLUE 211 

kataphoresis, precipitation, etc. According to this point of 
view the particles of the disperse phase are surrounded by a 
surface layer in which these ions are in much greater concen- 
tration than in the volume concentration of the dispersion 
medium. The hydrion and hydroxy 1 ion are particularly 
liable to such adsorption. In the case of a lyophile colloid, 
like gelatine, the charge may be either positive or negative, 
according to the nature of the predominant ions in the 
dispersion medium, and the amount of adsorption is deter- 
mined by the concentration of these ions in accordance with 
the adsorption law. 

In effect, therefore, the particles of the disperse phase 
each carry an electric charge of the same nature, and as 
similarly charged bodies repel one another, the particles of 
the disperse phase will tend to separate and to occupy a 
bigger volume. It is the author's opinion that this repulsion 
of similarly charged particles is the cause of the swelling of 
gelatine. The amount of charge and force — tending to 
swell — is due possibly to several ionic adsorptions, which 
may be considered to operate independently, and the power 
of repulsion is determined by the nett charge, which in the 
case of a " positive colloid " is positive, and in the case of a 
" negative colloid " is negative. As ions possess different 
electric charges, the charge on the disperse phase is subject 
to the valency rule. 

Now the repulsive force between two similar and similarly 

charged bodies is proportional to the amount of charge and 

is inversely proportional to the square of the distance between 

them. The amount of charge on a colloid particle will be 

determined by the dispersity — best signified by the specific 

surface (s) — and by the operation of the adsorption law 

1 
y=macn. The distance between the particles varies with 

the degree of swelling, and is determined by the cube root of 

the volume of the gel (v). Hence if F be the force tending to 

make the gelatine swell, we may write 

F=2, = -^ 



212 ANIMAL PROTEINS 

Now with all electrolytes, even with water, we have both 
positively and negatively charged ions, and y is consequently 
determined by the difference in the amounts adsorbed. 
Hence in the case of an electrolyte with an equal number of 

oppositely charged ionsy = ma x c n i — ma 2 c n %, where ay, a 2> and 
«!, n 2 , are the appropriate constants for the particular ions 
concerned. Hence at constant temperature, pressure, etc., 
we may write 

( L 'A 

Sm\CliCn — a 2 Cn) 

The force tending to make a piece of gelatine swell is pro- 
portional to its mass, which is perhaps fairly obvious. The 
swelling force is also an inverse function of the volume of the 
gel, and as swelling proceeds therefore the force tending to 
swell further decreases. The force tending to swell is pro- 
portional to the specific surface of the disperse phase, other 
factors being constant. To illustrate this one has only to 
imagine that one particle of the disperse phase be split into 
two particles each carrying half the original charge. It is 
clear that a new repulsive force becomes operative, which 
did not before influence the swelling, and that the distance 
between the particles is halved. In the swelling of gelatine, 
however, we may consider the dispersity constant for 
constant temperature, and if we consider unit mass we see 
that the force causing swelling depends upon the operation 
of the adsorption law and upon the degree to which the gel 
is already swollen. 

In the swelling of (say) one gram of gelatine to its 
maximum, both the contractile force of surface tension and 
the expanding force of electrical repulsion are in operation. 
At the commencement the latter is much the greater force — 
hence the rapid imbibition. Both these forces decrease in 
magnitude as the swelling proceeds, but the force tending to 
swell decreases at a more rapid rate, and the time comes 
when it has decreased to the precise value of the force tending 
to resist swelling. At this point equilibrium is established 
and the maximum swelling attained. Obviously this 



PROPERTIES OF GELATINE AND GLUE 213 
maximum will in many cases be determined largely by the 

value of ai&h - a 2 c*h. This factor, therefore, demands 
particular consideration. 

Now, unfortunately, the adsorption law constants for 
the different ions have not yet been numerically determined, 
so that we are still somewhat in the dark as to the operation 
of ionic adsorptions. It is possible, however, to form con- 
clusions of a qualitative or relative order, and these are such 
as to throw much light upon the question at issue. In the 
first place, we know that in general the various ions are not 
usually very widely different in the extent to which they are 
liable to be adsorbed. If this were otherwise, the valency 
rule would hardly operate so well in endosmosis, kata- 
phoresis, and precipitation. In consequence we must expect 
the differences between the ions to appear in small rather 
than in large concentrations, the amounts adsorbed being 
under those conditions more affected by changes in the 
volume concentration. At the larger concentrations, there- 
fore, the value of a x &h — a 2 c»2 is small, and the force causing 
swelling often tends to zero. 

There are, however, noticeable differences at lower con- 
centrations. Thus we know that if a substance be primarily 
a positive colloid, it will absorb kations more readily than 
anions. As gelatine falls into this class, we may therefore 
conclude that usually a Y > a 2 . Further, it often happens 
that very adsorbable substances are less affected by concen- 
tration changes, and in the case under consideration, there- 
fore, we should expect that n x > n 2 . Moreover, we know 
that the hydrion and hydroxyl ion are much more readily 
adsorbed than other ions, i.e. have a large value for a. 

Hence in the case of gelatine we expect that a^an — a 2 cn % will 
have a comparatively large value when one of the ions is 
H + or OH ~. Also we know that organic anions are usually 
much more strongly adsorbed than inorganic anions, and 
hence that in such cases a x is more nearly approached by the 
value of a 2 . It should be emphasized perhaps, at this point, 



214 



ANIMAL PROTEINS 



that these various considerations are not based upon any 
facts relating to the phenomena of imbibition in gels, or in 
gelatine in particular, but are based upon the behaviour of 
colloids in endosmosis, kataphoresis, electrolytic precipitation, 
adsorption, etc. 

Now if we select a few simple figures which are in accord 
with the above considerations, we can examine the value 

i_ _i_ 

of the factor a^cH — a 2 c n 3 in a purely illustrative and typical 




way, and at any rate form some idea as to the manner in 
which it is likely to vary. The figures might be : — 



Ion. 


n. 


0. 


Hydrion or hydroxylion 
Kation of a metal 
Organic anion 
Inorganic anion 


20 

15 

10 

6 


10 

7 
8 
6 



For the sake of simplicity we can assume that these ions 
are all monovalent. The ions adsorbed by unit mass will 
then be 10c™, etc. If these hypothetical adsorption iso- 



PROPERTIES OF GELATINE AND GLUE 215 

therms be plotted as usual we get the fairly typical curves 
shown in Fig. 1. 

Now in practice there are always two of these ions, each 
giving its own specific effect in opposite senses, and the 

difference (a^nl — a 2 %) represents the nett charge adsorbed. 
Hence we have the following combinations : — 



Inorganic acid 
Organic acid 
Alkali .. 
Inorganic salt . . 



ioc 2 ^— 6c^ 
ioc^—jc?* 



If we plot these values of nett adsorption against the 
concentration we obtain the curves shown in Fig. 2. 




On the assumption that the nett charge adsorbed is the 
dominant factor in determining the maximum swelling at 
equilibrium, one must therefore regard the curves of Fig. 2 
as representing the changes in volume of the swollen gel as 
the concentration is increased. Now in type these curves 
correspond to those obtained by experiment from hydro- 
chloric acid, acetic acid, caustic soda, and common salt. 
The maximum swelling with hydrochloric acid increases 
rapidly with the concentration at first and then rapidly 
decreases, though not at such a great rate. The swelling 
with acetic acid increases less rapidly and to a less maximum, 



216 ANIMAL PROTEINS 

but decreases more slowly. With common salt there is a 
slight swelling followed by contraction. Caustic soda gives 
a rapid increase in volume at first, afterwards much less so, 
and finally yields an exceedingly slow decrease. The corre- 
spondence of these facts with the type-curves inevitably 
suggests that the phenomenon of swelling might be accounted 
for, in part at least, along these lines. 

Of course it is not likely that the simple figures selected 
for the illustration of the argument are either relatively or 
absolutely correct. Thus we know that the adsorption 
curve for hydrions and hydroxylions are not likely to be 
quite identical, as assumed above. As gelatin is primarily 
slightly positive, it is probable that the values of a and of n 
for hydrion adsorption will be relatively slightly greater. 
The relative values supposed, however, are near enough to 
illustrate the contention that the type of the maximum 
volume curve can be explained on this assumption of different 
adsorption isotherms for each of the ions. 

If the remarks on the compression of the continuous 
phase be recalled, it will be obvious that in the present para- 
graphs we have been giving the question of equilibrium - 
volume a rather one-sided consideration. The volume of the 
gel when equilibrium is established may be determined in 
type by the nett charge adsorbed by the disperse phase, but 
it will be modified also by the lyotrope influence of the 
particular substance on the continuous phase. When 
gelatine swells in solutions the influences on both phases are 
always in operation, and either upon occasion may become 
predominant. In the case of neutral organic substances, 
such as cane-sugar, the lyotrope influence is the determining 
factor. In the case of neutral salts the predominant influence 
is decided by the place occupied by the salts in the lyotrope 
series. If at either end of the series the lyotrope influence is 
uppermost and the effect of ionic adsorptions is practically 
swamped. Thus sodium sulphate and sodium iodide hinder 
and promote imbibition respectively as could be expected 
from their strong lyotrope power. On the other hand, 
in the case of sodium chloride, which has comparatively 



PROPERTIES OF GELATINE AND GLUE 217 

feeble lyotrope influence, the relatively different adsorptions 
of its ions comes to the fore. With acids and alkalies the 
relatively large adsorption of the hydrion and hydroxylion 
causes this to be the predominant influence, but we must 
concede the possibility that purely lyotrope influences may 
be at work in some cases, and especially at the greater con- 
centrations. Indeed, it is sometimes a difficult problem to 
decide whether an increase or decrease in swelling is due to 
lyotrope or adsorptive influence, but, broadly speaking, we 
can expect strong lyotrope effects at either end of the series 
and also at large concentrations, and we can expect strong 
adsorptive effects in dilute solutions, in the middle of the 
lyotrope series and in the case of alkalies and acids. 

For much of the above explanation of the nature and 
behaviour of gelatine, the author must himself take 
responsibility, and in this section he has freely quoted from 
his own papers upon the subject (see References). He 
claims that his view of a gelatine gel as involving a network 
of compressed water, liable to modification by lyotrope 
influence upon the continuous phase and by ionic adsorptions 
of the disperse phase, is most in harmony with the recent 
advances in our knowledge of colloids ; that much of the 
theory is a necessary corollary of those discoveries ; and also 
that he has found this view to be a sound guide in practice, 
both in tanning and in gelatine manufacture. 

Many other theories have been advanced, but most are 
generalizations over too limited a field, and from experiments 
with only a few substances, and show little or no correlation 
with the wider facts of colloid behaviour. That of Procter, 
for example, discards altogether the idea of a two-phased 
structure of the gel as an " unproved and rather gratuitous 
assumption," dismisses surface tension considerations as 
" more complicated and less verified," and adsorption as 
" wholly empirical," whilst it ignores lyotrope influence and 
the analogy with agar gels completely. Procter's theory 
applies mainly to the swelling of gelatine by acids, which 
swelling he considers to be due to the osmotic pressure of 
the anion of a highly ionizable salt formed by the chemical 



218 ANIMAL PROTEINS 

combination of the acid with gelatine. On this assumption, 
mathematical considerations show that the electric charge 
on the gelatine is given by the expressions = \/^ex -f- e 2 , where 
2=the amount of ion taken up, x the concentration of the 
surrounding solution, and e the excess concentration of 
diffusible ions in the jelly. 

The property of gelatine and glue which is chiefly used 
in classifying them into grades of different commercial value, 
is the strength of the jelly obtained as compared with any 
arbitrary standard gelatine. An enormous number of other 
physical tests have been devised, but none are nearly so 
simple or so reliable. Gelatine is unfortunately very liable 
to hydrolysis even by water, and long before any amido- 
acids, etc., have appeared there is a change to a not greatly 
hydrolyzed product (sometimes called /J gelatine) which 
has lost the power of setting to an elastic gel. It is thus the 
lyophile nature which has been altered, and the fall in 
elasticity corresponds to the fall in power of compressing 
water, which is proportional to the concentration of a gelatine. 
Now the elasticity of a gelatine gel varies as the square of 
the concentration. Hence if one so arranges the concentra- 
tions of standard and unknown samples that gels of equal 
elasticity are obtained, the concentration of a gelatine is the 
same in both gels, and the relative amounts of a gelatine in 
the original samples are inversely proportional to the weights 
used to give gels of equal elasticity. The " strength " of a 
gelatine or glue is therefore usually stated as the number of 
grams of a standard gelatine which will yield a gel with 
elasticity equal to that from ioo grams of the gelatine or 
glue being tested. Elasticity is matched by lightly pressing 
with the finger-tips. 

It is also possible to grade samples of gelatine and glue 
by the estimation of " peptones," whose amount indicates 
the degree of hydrolysis. Nitrogen is estimated by Kjeldahl's 
method in the sample and in the precipitate obtained by 
saturating a solution with zinc sulphate. The difference is 
calculated as peptones by muitipiying by 5 "33. Trotman 
and Hackford say that the results are in the same sequence 



PROPERTIES OF GELATINE AND GLUE 219 

as those of the finger test. The method, however, is much 
more laborious than the " finger test." 

Gelatine is also graded according to the results of bleach- 
ing and clarifying, but with quite arbitrary standards, 
largely determined by the fancy of the customer. 

Chemical analyses, involving estimations of ash, lime, fat, 
acid, water, insoluble matter, and poisonous metals, e.g. 
arsenic, copper, zinc and lead, are of value for special cases 
according to the destiny of the goods. Special physical tests, 
such as " breaking strain " and " foam test," are also of some 
little value in special cases. 



REFERENCES. 

" The Chemistry of Colloids," W. W. Taylor. 1915. 

" Handbook of "Colloid Chemistry," W. Ostwald. 191 9. 

" Chemistry of Colloids," Zsigmondy and Spear. 191 8. 

" Introduction to the Chemistry and Physics of Colloids," E. Hatschek. 

" Surface Tension and Surface Energy," Willows and Hatschek. 

" Chemistry of Colloids," V. Pose hi. 

" Grundziige d. Dispersoid Chemie," von Weimarn. 

" The Lyotrope Series and the Theory of Tanning," Bennett, J.S.L.T.C., 
1917, p. 130. 

" The Swelling of Gelatine," Bennett, J.S.L.T.C, 1918, p. 40. 

" The Swelling of Gelatine," Procter, J.C.S. Trans., 1914, 105, 313 ; and 
Koll. Chem. Beihefts, 191 1, 2, 234. 

" The Swelling of Gelatinous Tissues," Procter, J.S.C.I., April 16, 1916. 

" Summary of Procter's Views, and Bibliography," Collegium (London), 
p. 3, 1917. 

" Lyotrope Influence and Adsorption in the theory of wet work," Bennett, 
J.S.T.C., 1920, p. 75. 

For the " ringer test," see — 

" Glue and Glue Testing," Rideal, 2nd ed., p. 158. 

" Leather Trades' Chemistry," Trotman, p. 241. 



Section II. —RAW MATERIALS AND PRE- 
LIMINARY TREATMENT 

The raw materials for the manufacture of gelatine and glue 
may be classified according to their origin. The preliminary 
treatment, which comprises chiefly purifying and cleansing 
operations, is varied according to type of manufacturing 
process for which it is a preparation. 

In the case of hide or skin gelatine, the raw material is 
a bye-product of the leather industry. After the hides or 
skins have passed through the preparatory processes which 
convert them into " pelt " (see Part I., Section II.), they are 
so trimmed that all that is left will make a useful leather. 
These " trimmings " or " roundings " include ears and 
noses, the udders of cows and heifers, and also include parts 
from the butt, belly and shanks which are collective^ termed 
"pieces." The operation of fleshing (Part I., Section II., 
p. 22), in which fat and flesh are cut from that side of the 
hides and skins which was next the flesh, also involves 
cutting into the collagen to some extent, and these " flesh- 
ings " comprise another very large class of raw material. 
The fleshings obtained by hand labour contain distinctly 
more hide substances than those obtained by machine work, 
and their commercial value to the gelatine manufacturer is 
of course proportionate to the collagen content. Some hides 
and skins are split in the pelt (Part I., Section IX. ; Part II., 
Sections II., III. and IV.), and the " flesh split," though 
sometimes made into leather, is also used in making gelatine, 
a high quality being obtained from such material. Minor 
sources of material are tendons and cartilages, and also hides 
and skins which have been too much damaged by partial 
putrefaction or by accidents to make sound leather. Of 
course the material from the hides for heavy leathers form 



GELATINE AND GLYCERINE 221 

the greater bulk of the raw material for skin gelatine, which is 
thus derived principally from ox hides, but sheep and goat 
skin pieces have also an important place. The skins of 
other animals, such as dogs, cats, hares, rabbits, not usually 
made into leather can also be depilated and used for making 
skin gelatine and glue. Horse hide fleshings and pieces are 
sometimes used, but are notorious for the poor quality of 
their product. They seem to contain less a gelatin. All 
these materials are of course readily putrescible and must 
be put " into work " without much loss of time. When 
it is impossible to convey them from the tannery to the 
gelatine factory quickly enough, e.g. foreign material, the 
"glue stock " is dried out completely and sold in that con- 
dition. In the manufacture of pickers from limed pelt there 
is some superfluous material, and this is cut into shavings 
and dried. This " picker waste " also forms a useful source 
of raw material. Skin gelatine material is not very strong in 
gelatine-substance. The fleshings, pieces, etc., contain much 
water, even up to 80 per cent. This, however, is very 
variable, and only a practical test or a hide substance deter- 
mination can indicate the commercial value of any particular 
material. This value, moreover, is determined not only by 
the yield and quality of the gelatine which can be obtained, 
but also by the yield of grease, the valuable bye-product. 

The preliminary treatment of material for skin gelatine 
consists essentially of liming and of washing. The object 
of each process is to purify. Liming has much the same 
action on. hide pieces, etc., as on hides, and indeed the 
liming treatment is somewhat superfluous on cuttings from 
well-limed hides. The material is plumped up and the 
partially hydrolyzed products are taken into solution. Lime 
also acts as mild antiseptic, stops any putrefaction and 
liberates ammonia formed by fermentation in transit to 
the factory. When plumping is particularly wanted (as in 
wetting in dry stock) caustic soda is sometimes used as an 
assistant (cf. dried hides, p. 18). Sodium sulphide has 
also been used for this purpose. The liming is in brick pits, 
an excess of undissolved lime being always used. It is 



222 ANIMAL PROTEINS 

advantageous frequently to disturb or agitate the goods 
in the lime pits. Up to ten weeks liming has sometimes 
been given, but about three weeks is now generally con- 
sidered sufficient, and the tendency is to shorten the time. 
The lime and soda have also a detergent action on soiled 
stock, and they probably assist in hydrolyzing the pigments 
of the hair roots and sheaths. They also saponify and 
emulsify the grease, and it is obvious, therefore, that liming can 
be carried too far. Slaked lime, of course, must always be used. 
After liming the soaked, softened and plumped stock is 
washed as thoroughly as possible. To do this it is necessary 
to supply repeated batches of clean cold water. Some 
manufacturers, however, use the warm water from the 
evaporators. Wooden vats or brick pits with arrangements 
for agitation, for draining off and for inspection, are used for 
this purpose. The agitation may be carried out by means 
of revolving shafts or drums with projecting curved spokes 
or vanes. An American patent (Hoeveler's glue stock 
washer) involves the use of a paddle wheel. It is combined 
with a settling tank to gather particles of stock. In the 
washing the chalk, excess lime, dirt, etc., are quickly removed 
and a slow deliming process is commenced. The sediment 
from the washers and wash waters has some value in making 
fertilizers. Deliming cannot be carried on further than 
certain limits by water alone. Hence acid is often added to 
finish off the process. Hydrochloric acid has the advantage 
of forming soluble salts, but if they are not removed com- 
pletely their lyotrope influence is to weaken the gelatine. 
Sulphuric and sulphurous acids are even cheaper, and the 
lyotrope influence of their salts is in the opposite sense. 
The latter also has the advantage of destroying sulphides, 
an important advantage for food gelatines. Whatever acid 
is used, however, it is evident that an abundance of pure 
cold water is the fundamental requirement of a pure product. 
It is a sound maxim in gelatine manufacture to avoid, if at 
all possible, the addition of any soluble substance, for it is 
always present in a more concentrated state in the finished 
article. Thus if its solubility be even moderate, one is 



GELATINE AND GLYCERINE 



223 



likely to attain supersaturation in the " cake " and con- 
sequently a dull product. Further, lyotrope influences can 
never strengthen a gel very much, but may and often do 
weaken it very considerably. Hence the aim of most 
manufacturers in the preliminary treatment is so to delime 
that a nearly neutral and salt-free product is obtained. An 
exception is the case of skin gelatine in which excess of 
sulphurous acid is used. This process has for its object not 
only deliming and purifying, but also a bleaching action. 

In the case of bone gelatine, the raw material is such 
that there are much longer and more elaborate preparatory 
processes. This arises from the fact that about half the 
bones of animals consists of mineral matter, chiefly calcium 
phosphate. Bones, of course, vary in composition to some 
extent, and those from younger animals contain distinctly 
less of the mineral constituents. Approximately speaking, 
bones have the following average composition : — 



Gelatinous matter 


. . 21 J per cent 


Fat 


•• 12J „ „ 


Calcium phosphate 


• • 48 „ „ 


Calcium carbonate 


• • >> >> 


Alkali salts, silica, etc. 


.. 2| „ „ 


Water 


•• I2j „ „ 




100 „ „ 



It will be seen, therefore, that the manufacture of bone 
gelatine and of a comparatively large proportion of phos- 
phate involves the recovery and purification of much fatty 
matter. The manufacturing processes are naturally subject 
to considerable variation. One respect in which they differ 
is the stage in which grease is removed. Sometimes this is 
simply done as the need and occasion arise, and it is skimmed 
out in the acid or water extractions, but it is now more usual 
to have a special " degreasing " process. There are, more- 
over, two quite distinct types of manufacture. In one of 
these (the boiling process) the routine bears some resemblance 
to that for skin gelatine. In this process the bones are 



224 ANIMAL PROTEINS 

washed and cleansed and then immediately subjected to 
extraction with water. This removes the gelatinous matter 
and leaves the phosphate and earthy matters behind. Grease 
may be removed before the water extraction, but is also 
sometimes removed by skimming off during the extraction, 
as is usual in the case of skin gelatine. This procedure is 
now not much favoured unless only a low-grade glue is 
required. In the other type of manufacture (the acid 
process) the material is first degreased, and then the mineral 
matter is extracted or dissolved by acids, leaving the 
gelatinous matter behind for subsequent refinement and 
solution. The acid process has long been preferred for high- 
class bone gelatine, and hence needs further discussion. 

The degreasing operation was once brought about by 
steaming only, but is now accomplished with the assistance 
of fat solvents. 

The object of cleansing is not only to remove dirt, but 
also fleshy matter which often adheres to the bones. This 
may contain a little gelatine, but consists mainly of other 
proteins and insoluble fibre, neither of which are wanted 
in the water extraction. The mill consists of a large cylinder 
of stout wire gauze. This revolves round the axis of the 
cylinder, and the bones are fed in at one end by a hopper and 
are discharged at the other. The revolution of the mill 
causes the friction which polishes off the fleshy matter. The 
dirt and flesh fall through the gauze and are sent to the 
fertilizer factory. The polishings are sometimes further 
separated by a similar machine. Raw bones may thus yield 
nearly 60 per cent, of degreased bones, and about 56 per 
cent, cleansed bones ready for extraction, and 3 or 4 per cent. 
" bone meal." 

The next stage is the extraction of the mineral matters 
by acid, for which purpose hydrochloric acid has proved 
very suitable, as both phosphate and carbonate of lime are 
dissolved by it. The usual counter-current system of 
extraction is used [cp. Reaching and extract manufacture, 
Part I., Section III., p. 35]. The process is methodical 
and regular, the acid liquor passing successively through a 



GELATINE AND GLYCERINE 225 

battery of six vats in such a manner that the liquor richest 
in lime salts comes into contact with the bones most recently 
charged ; the fresh acid thus acts upon the nearly extracted 
bones. The hydrochloric acid used is of 8 to 10 per cent, 
strength (5 to 7 Be.). Stronger acid is apt to hydrolyze 
(" rot ") the gelatine, whilst weaker acid takes longer time. 
The process takes 8 to 10 <\.a.y§, though up to 14 days is 
sometimes given, and, on the other hand, the process has 
been occasionally reduced to 4 days. The gelatinous matter 
undissolved has the shape of the original bone, but is much 
swollen. When the acid liquor is saturated with lime salt, 
the liquor is drawn off from below the vats sent to the 
phosphate precipitation tanks. The phosphate is usually pre- 
cipitated by adding just sufficient milk of lime to neutralize 
the hydrochloric acid. The precipitated phosphate is then 
well washed by decantation to remove calcium chloride. It 
is then drained, and dried at a low temperature. As a large 
bulk of phosphate is obtained it is often filter-pressed and 
dried quickly in long revolving chambers through which a 
current of air is passed. The phosphate is sometimes also 
precipitated by ammonia. It is then more easily washed and 
dried, and the ammonium chloride is recovered and may 
be used to regenerate ammonia, or be sold as a valuable 
bye-product. Sometimes the acid liquor is not used for 
making precipitated phosphate, but is evaporated with 
animal charcoal and silica and then distilled to make phos- 
phorus. 

The next stage is the purification by washing of the 
gelatinous matter which remains. The vat is filled up with 
pure cold water and the material allowed to steep for six or 
seven hours. The acid and salts remaining diffuse outwards 
into the water. This is drained off and replaced by fresh 
water, and the procedure repeated half a dozen times or 
as often as necessary. The end is said to be determined by 
the absence of a precipitate on adding silver nitrate to the 
wash water, or by the absence of any action on blue litmus 
paper. It will be seen, however, that there are two actions 
involved, one being the removal of calcium chloride and the 
B. 15 



226 ANIMAL PROTEINS 

other the removal of excess acid. The former is the easier, 
and is almost necessarily brought about by the latter. Hence 
in some factories the neutralization is brought about, there- 
fore, by the addition of a certain quantity of soda, or more 
usually by lime, and the material is sometimes submitted 
to a veritable liming by which it remains in milk of lime for 
about three weeks, the lime liquor being renewed several 
times. The product is finally washed again to remove excess 
lime. This is carried out in a rotating vessel through which 
passes a continuous stream of water. If a slightly acid 
gelatine is required, however, the lime and liming are both 
superfluous, and the procedure is simply to wash as 
thoroughly as possible and then to immerse the material in a 
i per cent, sulphurous acid solution for 3 hours to bleach, 
and then to proceed with the water extraction or solution of 
the gelatine. The hydrochloric acid used for these processes 
should be as pure as possible, and the degreasing as thorough 
as possible, for, if not, a gelatine with a bad odour is liable to 
be obtained. 

Instead of using hydrochloric acid for the solution of 
mineral matter, sulphurous acid is sometimes employed, and 
has the advantages that its bleaching effect is thereby 
obtained throughout the process, and that it is recoverable 
for subsequent use. The Bergmann process, most generally 
favoured, is described very concisely by Rideal thus : "A 
sulphurous acid solution is made to circulate over the bones 
in a series of closed tanks, the solution being continually 
enriched with sulphurous acid from a cylinder of the liquefied 
gas. The resulting liquor, containing an acid calcium phos- 
phate and calcium bisulphite, is heated by steam in a leaden 
digestor, when the excess of sulphurous acid is liberated and 
passes back to the tanks, while neutral calcium phosphate 
and sulphite are precipitated. The latter is decomposed 
by an equivalent of hydrochloric acid, setting free the 
remaining sulphurous acid, which is returned to the tanks, 
leaving calcium chloride in solution, and neutral calcium 
phosphate in suspension." Not more than 5 per cent, of 
sulphurous acid is said to be lost in this process, and the 



GELATINE AND GLYCERINE 227 

gelatine is more thoroughly bleached. It is subsequently 
well washed before extraction. 

Recovery and Purification of Grease.— The de- 
greasing operation, which is applied usually to bones (p. 
224) and to skin glue scutch, was once brought about by 
steaming only, but is now accomplished with the assistance 
of fat solvents, though in the latter case steaming together 
with mechanical centrifugal force has proved sufficiently 
successful. On the Continent carbon disulphide was once 
largely used as solvent, and in this country benzene has 
been employed, but their low volatility and high inflamma- 
bility, as well as their expense, make both these substances 
somewhat unsuitable, and it is now usual to make use of 
petroleum oils, whether Scotch, American or Russian. A 
fraction which boils about the same temperature as water is 
usually employed, and all of it must be volatile under 
280 F. Before the actual grease extraction the bones 
should be sorted over and unsuitable substances (horns, 
gravel, iron, etc.) removed. They are also usually put 
through a mill and roughly crushed or broken. The actual 
grease extraction plant consists of large copper vessels which 
will each take 5 tons of bones. These extractors are 
arranged in sets so that the degreasing is proceeding in some 
whilst the others are being emptied and recharged. The 
doors for charging and emptying must be securely fastened. 
When the extractor is charged the solvent is run in and heated 
by a steam coil which eventually causes it to distil. After 
some hours the remainder, which has dissolved much grease, 
is run off, and a fresh lot of solvent is added and heated up. 
After four such extractions only about £ per cent, of grease 
remains in the bones. To remove the remainder of the 
solvent high-pressure steam (80 lbs.) is blown through the 
bones. The extractor is then opened and the degreased 
and somewhat dried bones are mechanically conveyed to 
the cleansing mill. The grease solutions obtained are 
subjected again to steam with a view to removing the 
solvent and obtaining it for repeated use in this sense. 
The efficient distillation and recovery of the solvent 



228 ANIMAL PROTEINS 

is indeed an essential element in the success of the 
process. 

The greases obtained, whether by the use of fat solvents 
or by skimming off during extraction, or in any other way, 
are mixed together as is appropriate to their origin and 
purity, and subjected to further puiification, the object of 
which is to remove gelatinous and albuminous matters, and 
to decompose lime or soda soaps. The precise methods of 
purification are, of course, dependent mainly upon the 
impurities known to be present, but the readiest method is 
to give the grease further steaming or boiling with water, 
and so effect by washing and by solvent action the elimination 
of non-fatty matters. In many cases it is found advan- 
tageous to employ mineral acids or oxidising agents to assist 
the process. The process may be repeated as often as is 
desired. 

The recovered and purified greases are often of a high 
standard of purity, and the best are quite fit for edible pur- 
poses. The large extension of the margarine industry in this 
country has indeed caused a larger proportion than ever of this 
bye-product to be so used. In some cases it is found com- 
mercially advantageous to submit the grease to action of the 
filter press, and so to separate it into solid and liquid portions, 
the former containing a much larger proportion of stearin, 
and the latter of olein. Much of the grease from the gelatine 
trade is also found suitable for soap manufacture, and is 
therefore a valuable source of glycerine. 

Other Raw Materials. — Whilst hide pieces and flesh- 
ings, and animal bones, comprise the principal raw material 
for the manufacture of gelatine and glue, there are also 
minor sources of raw material which, though often not 
suitable for gelatine manufacture, will yield a satisfactory 
glue. Thus the skins, bladders and bones of fish form the 
source of " fish glue." Sole skins, indeed, when deodorized 
by chlorine and decolorized by animal charcoal, are 
made into gelatine. The bladders of some fish (e.g. the 
sturgeon) are washed, purified and dried with rolling to 
make " isinglass," a form of natural gelatine in which 



GELATINE AND GLYCERINE 229 

the original fibrous structure is retained. There is a 
limited demand for this material for clarifying purposes by 
brewers, wine merchants and cooks. 

Leather waste may sometimes be used to make a low- 
grade glue. Vegetable-tanned leather offers much difficulty 
unless very lightly and recently tanned. The tannage must 
be stripped by drumming with weak alkalies, e.g. borax, 
sodium sulphite, or weak soda. Chrome leather may be 
stripped easily and completely by Rochelle salt and other salts 
of hydroxy acids (Procter and Wilson), and also by ammonia 
acetate, oxalate and similar salts (Bennett), also by certain 
organic acids (Lamb). Processes are patented by which 
chrome leather is digested with lime to make glue, the 
chromium hydrate being insolubilized. Viscous and 
tenacious substances are also obtained from some vegetable 
matters and are called " glue." 



REFERENCES. 

" Glue and Glue Testing," S. Rideal, D.Sc, 2nd ed. ; Skin Gelatine and 
Glue, pp. 25-48 ; Bone Gelatine and Glue, pp. 59-66. 

" Gelatine, Glue and their Allied Products," T. Lambert, pp. 11-52. 
" Encyclopedic chimique," Fremy, tome x. 



Section III.— EXTRACTION 

The term " extraction " is applied to that essential process by 
which the gelatinous matter from whatever raw material is 
used, is actually dissolved in water and removed from the 
rest of the material. Extraction is often termed " boiling " 
or " cooking." Whether one is treating hide fleshings and 
pieces or whether one is dealing with raw or acidulated bones, 
the general principles of extraction are much the same, and 
most of this section is equally applicable to any class of 
material. 

The chief principle of extraction is so to arrange the 
process that both the material and the extracted liquor are 
maintained at high temperatures for the shortest possible 
time. As we have observed, gelatine is readily hydrolyzed 
by hot water, and as hot water is needed for its extraction 
or solution, care must be taken to remove the solution as 
soon as possible from the source of heat. In practice this 
can only be done somewhat imperfectly, as it is necessary 
to obtain a gelatine sol of several per cent, strength before 
removing it from the extraction vessel. The stronger this 
sol is made before removal, the less the time, trouble and 
expense is incurred in evaporation subsequently, but the 
more is the exposure to heat with consequent weakening 
of the gelatine. Hence in practice it is necessary to com- 
promise. The matter is complicated further by the necessity 
of obtaining a clear sol, for which it is desirable that the sol 
obtained in extraction should not be too concentrated, as 
impurities settle and filter much more readily from weaker 
and less viscous sols. 

It will be understood, therefore, that whatever material 
is being extracted, the most favoured procedure is to extract 



EXTRACTION 231 

in fractions. The first fraction, which is least exposed to 
hydrolytic decomposition, produces the highest quality 
products, and the subsequent tractions (nearly always two 
more, and sometimes several) yield products which gradually 
become of inferior quality owing to the number of times the 
raw material has been re-heated. 

Within limits, the precise temperature of extraction does 
not have the importance one would expect. L,ambert 
suggests the temperature of 185 F. as suitable for both 
skin and bone gelatine, and most manufacturers would, on 
the whole, endorse this. If, however, a higher temperature 
be preferred, the hydrorytic action is increased in intensity 
but decreased in its time of operation, whilst if a lower 
temperature be adopted the decomposition is retarded in 
speed, but is increased in totality because of the longer time 
needed to obtain a suitable strength of liquor. Thus, with 
care, much the same result is obtained by extraction at near 
boiling-point for a short time as by extraction at 160 F. for 
a long time. The higher temperatures have the definite 
advantage of speed, whilst the lower temperatures have the 
advantage that one may choose to be satisfied with a weaker 
extract, and so gain a little in the strength of the gel, by 
throwing more work on the evaporator. One other point 
should, however, be borne in mind in this connection, viz. 
that a gelatine sol kept at temperatures above 185 F. begins 
to deteriorate in colour. Whilst, therefore, much depends 
upon the precise class of material, it is broadly true to say 
that the higher temperatures are advantageous for glue, 
whilst the lower temperatures are preferable for the highest 
quality gelatine. 

Extraction in open vats is used both for skin and bone 
gelatine. It is usually preferred when it is intended to 
extract at the lower temperatures, and it is usually adopted 
also when the material is such that the extraction is com- 
paratively rapid, as for example in the case of skin gelatine 
and bones by the acid process. The vats themselves are 
often constructed of wood, in which case they are heated 
by a copper (or brass) steam coil. They may be constructed 



232 ANIMAL PROTEINS 

also of iron, cast or wrought, the former being cheaper, less 
liable to corrosion, but more liable to fracture. In the case 
of iron vessels the heating may also be done by a steam coil 
beneath a false bottom, but it is sometimes arranged that 
iron vats are heated by a steam jacket, and even by a hot- 
water jacket. Heating in either wood or iron vessels has 
been brought about by direct application of raw steam, but 
the results are both uncertain and unsatisfactory owing to 
local overheating. Whatever appliances are used agitation 
of the material or liquor is advantageous. 

Extraction in closed vats is also used. This is generally 
associated with extraction at higher temperatures, and more 
often also with the manufacture of glue than of gelatine. 
It has been used on the Continent for skin glue, and in this 
country for bone gelatine and glue by the " boiling " process. 
In this system of working the vessels are usually made of 
|-inch steel plates, and will take a charge of 3 to 5 tons of 
material. It is claimed for the system that there is a lessened 
steam consumption as well as lesser manipulation, that strong 
liquors are more easily and quickly obtained, and that 
the material may be more thoroughly exhausted. Extrac- 
tion is sometimes made by steam and water playing alter- 
nately on the material, but many manufacturers prefer the 
use of direct steam, keeping the pressure at 15 lbs. for about 
2 hours. The pressure is then reduced considerably and the 
process finished off by spraying the material with water. 
From such a procedure a 20 per cent, glue sol may be obtained. 
It is common to work such extractors in couples or in 
batteries of four to six. It will be readily understood that the 
process is suitable for making bone glue when the phosphate 
has not been dissolved. The high temperature is in this 
case almost necessary to ensure thorough extraction. It 
will be equally clear that the process is not so suitable in the 
manufacture of a strong gel. 

As alternatives to the systems of fractional extraction, 
several processes have been devised in which the extraction is 
continuous. 

Amongst these is the tower system, in which the material 



EXTRACTION 233 

is placed upon a series of perforated shelves arranged inside a 
steam-tight cylinder or tower. Water is admitted from the 
top and trickles down over the material whilst steam is 
admitted from the bottom. Superheated steam is sometimes 
used. The material may thus be digested with a minimum 
amount of water, and the sol passes out of the apparatus 
and from the action of heat soon after it is formed. From 
bones the sol obtained is of good colour, but is somewhat dull. 
Several variants of this process have been patented. 

Another continuous system of extraction is that involving 
the use of the Archimedean screw. The material is fed into 
one end of a cylinder carried along and discharged at the 
other end by the screw. The cylinder is of metal gauze and 
is steam jacketed. (L,ehmann's patent, 1912.) 

Continuous systems, involving a battery of digestors 
connected by pipes, have also been devised. Arrangements 
are made of course for admitting water and steam as required. 



REFERENCES. 

'* Glue and Glue Testing," by S. Rideal, D.Sc, 2nd ed., pp. 47-56 and 61. 
" Gelatine, Glue and their Allied Products," by T. Lambert, pp. 21-24, 
40, 42-44, 49 and 51. 

" Encyclopedie chemique," Fremy, tome x., p. 83. 



PATENTS. 



Edison : U.S.A. patent, 1902, 703204. 
Bertram : English patent, 1892, 951. 
Dorenburg : German patent, 1911, 239676. 
Lehmann : French patent, 1912, 441548, 



Section IV.— CLARIFICATION AND 
DECOLORIZATION 

After the raw material has been appropriately prepared 
and an aqueous extract or gelatine sol obtained therefrom, 
there are certain refinements necessary before the weak sol 
is evaporated. These purifying processes include (i) clarifi- 
cation, (2) decolorization, and (3) bleaching. Whilst most 
manufacturers have more or less successfully solved the 
problems involved in these processes, the practical methods 
that are in common use have been evolved and elaborated 
in a purely empirical way, and the underlying principles have 
been very imperfectly recognized, and indeed often confused 
and misunderstood. Hence it is even yet not uncommon 
to find these terms rather loosely used, and it is one aim of 
this section to define and distinguish these various operations 
in principle as well as in practice. 

Clarification consists essentially in the removal of 
suspended matters, with the consequent production of a sol 
or gel which is bright, clear, and apparently homogeneous. 
Bleaching consists essentially in destroying the colouring 
matters of the sol by chemical action, such as oxidation or 
reduction. Decolorization involves the removal rather than 
the destruction of colouring matters, and does not therefore 
imply a chemical action in the ordinary sense. 

Clarification may be now considered more particularly. 
It is necessary in this connection to consider what is meant 
by " suspended matter." The modern view is that the 
difference between a true solution and a muddy liquor or an 
emulsion is one chiefly of degree. If the particles of matter 
in suspension or emulsion (the disperse phase) be reduced 



CLARIFICATION AND DECOLORIZATION 235 

in size they eventually merge into colloidal sols which are 
sometimes analogously named " suspensoids " and " emul- 
soids," if further reduced in size into " suspensides " and 
" emulsides," and with further reduction into true solutions. 
On this view not only suspensions and emulsions, but also 
sols, solutides and solutions are all heterogeneous. Now in 
practice the clarifying of a gelatine sol involves only the 
removal of the particles which are evident to sight. What 
is needed is that the product should make a sol or gel which 
to the naked eye appears to be optically clear both to 
reflected and to transmitted light. If desired, the limit 
could be expressed in terms of dispersity or specific surface. 
Now it is a comparatively easy matter to remove the coarser 
substances which often pass into the sol, e.g. undissolved 
portions of raw material or the insoluble portions, such as 
the hair, the grain (hyaline layer), and the elastic fibres of 
skin gelatine material, and the fibres which even remain in 
extracting acidulated bones. A more difficult proposition 
is the removal of still finer particles which may be almost 
said to be in colloidal solution, but which at any rate are so 
large that they cause a visible opalescence or even a turbidity 
of the gelatine sol. A more difficult task also is the removal 
of minute particles of grease, which are an exceedingly 
common cause of turbidity and which are often very 
effectively emulsified in the sol. 

Now at this stage it is necessary to point out that besides 
the difference in the size of the particles of the disperse 
phase, there is another important difference involved, viz. 
that the particles of a colloid sol carry an electric charge 
owing to the adsorption of electrically charged ions of the 
electrolytes (salts, acids or alkalies) present. If this charge 
be removed the colloid is precipitated (coagulated, floccu- 
lated) and is then filtered off with comparative ease. This 
precipitation can be brought about by a reduction or 
elimination of the potential difference between the disperse 
phase and the continuous phase. The electric charge given 
by the adsorbed ions may be reduced by dilution, for dilution 
causes a lessened adsorption of the charging ions. Hence 



236 ANIMAL PROTEINS 

the well-known practical fact that it is more satisfactory to 
filter a dilute gelatine sol. Further, the electric charge may 
be reduced also by causing the adsorption of an ion of 
opposite charge. This is the principle underlying the 
precipitation (of any colloid) by adding electrolytes. It is 
essential here to consider which ions are most likely to be 
adsorbed, and also to bear in mind what charge they carry. 
Now the hydrion (H+) of acids and the hydroxyl ion (OH~) 
of alkalies are most strongly adsorbed, so that to precipitate 
a negative sol, acid is very effective, whilst with a positive 
sol an alkali is an appropriate precipitant. Further, it is 
known that organic ions are usually more strongly adsorbed, 
hence when precipitating from an alkaline sol (negative sol), 
one should preferably select an inorganic or mineral acid 
rather than an organic acid. Thus in clarifying an alkaline 
gelatine sol, hydrochloric or sulphuric acid is to be preferred 
to acetic or lactic acid. Again, it is necessary to remember 
that a divalent ion carries twice the charge of a univalent ion, 
hence the precipitating power of an electrolyte depends upon 
the valency of the ion whose electric charge is opposite to 
that on the sol (Hardy's valency rule). Thus a negative 
sol is most easily precipitated by a monobasic acid. Thus 
hydrochloric acid is better than sulphuric, on account of the 
stabilizing effect of the divalent SO4 — ion on a negative sol. 
In such a sol, also, the valency rule indicates that the multi- 
valent kations, e.g. iron, Fe+ + + ; chromium, Cr+ + + ; and 
aluminium, A1+ + + , should have great precipitating and 
clarifying effect. This of course is known to be the case, 
aluminium salts having long been used. The rule indicates, 
also, that aluminium chloride would be better than the 
sulphate or than potash alum. Another feature of precipita- 
tion worth}' of mention is the phenomenon of " acclimatiza- 
tion." This describes the fact that when the precipitating 
reagent is added very slowly, or a little at a time, a larger 
amount must be used, and the slower the addition the greater 
the excess required. Hence in precipitating matters from 
an alkaline gelatine sol the acid, if practicable, should be 
added all at once. In any case it is clear that one should 



CLARIFICATION AND DECOLORIZATION 237 

aim at filtering a gelatine sol when it is near the iso-electric 
point, which is stable enough for gelatine itself, but a point 
of instability for many undesired impurities. Yet another 
phenomenon of colloid chemistry is concerned, viz. " pro- 
tection." The particles it is desired to precipitate not only 
adsorb ions of electrolytes, but also the gelatine sol itself, 
and the particles, thus covered by a layer of a stable emulsoid 
sol, attain much of the stability of this gelatine sol. Un- 
fortunately for gelatine manufacturers, gelatine possesses 
very great powers as " protective colloid," and this no 
doubt greatly enhances the practical difficulty of obtaining 
a clear and bright sol or gel. Here again dilution of the sol 
reduces the adsorption and correspondingly reduces, to some 
extent, the difficulty. 

With regard to the turbidity or opalescence in a gelatine 
sol due to minute globules of grease, the case presents some 
analogy to the coarser colloid solutions, but the analogy has 
its limits, for an emulsion of grease is not an emulsoid sol. 
Doubtless the grease globules exhibit adsorptive phenomena, 
in which case the valency rule comes into force ; the gelatine, 
also, by lowering interfacial tension, assists in protecting the 
emulsion ; but grease emulsions are certainly stabilized in 
alkaline media (hence the detergent effect of soap, soda, 
borax, etc.), and it is undoubtedly easier to separate the 
emulsion by making the medium acid. Hence the practical 
fact that an acid sol is more easily clarified from grease than 
an alkaline or even than a neutral one. 

The next stage in clarification is the separation of pre- 
cipitated matters and of the coalesced particles of grease. 
This may be attained by the two processes usual. in such a 
problem of chemical engineering, viz. sedimentation and 
filtration. After precipitation, therefore, the sol should be 
allowed to stand for some hours, during which time the 
precipitate not only flocculates but also settles to the bottom, 
and the globules of grease coalesce further and rise to the 
top, from which they may be skimmed off. Sedimentation 
alone is both too slow and too incomplete to be sufficient 
for proper clarification, and in these days it is always 



238 ANIMAL PROTEINS 

supplemented by the use of the filter-press. This well- 
known appliance can easily be adapted to the local require- 
ments of the manufacturer. As speed of working is an 
essential requirement it is necessary to have a large filtering 
surface, and this may be done either by increasing the 
number of plates in the press or by increasing the area of the 
plates used. The large plates, however, are often cumbrous 
and inconvenient, and if of metal are very heavy. The plates 
may be constructed of well-seasoned wood, or in the case of 
alkaline gelatine and glues, even of iron. The framework 
is in any case usually iron. Acid gelatines and glues may 
have wooden plates, but " acid-proof " alloys are sometimes 
used to make them. Where it is essential to filter quickly 
two presses may be arranged in parallel, thus doubling the 
active filtering surface. When it is essential to obtain the 
highest possible clarity, two presses may be worked in series, 
which, in effect, means that the sol is filtered twice. In 
using the filter press for gelatine and glue it is most necessary 
to observe the most scrupulous cleanliness, and the plates 
must be frequently washed and sterilized. Rideal recom- 
mends weak chlorine water or bleaching powder solution 
for this purpose. 

The process of decolor iza lion, by which colouring matters 
are removed without being chemically altered or destroyed, 
u.ually precedes or takes place concurrently with the filtra- 
tion. The underlying principle of this operation is adsorp- 
tion. The colouring matters are usually in colloidal solution 
and most frequently are emulsoids, hence they are substances 
which are known to be exceedingly susceptible to positive 
adsorption. It is probable, also, that in a gelatine sol are 
particles which cause turbidity, though not coloured, and 
which are capable of being adsorbed. Hence the adsorption 
of colouring matters not only makes the sol more colourless, 
but in all probability makes it brighter and clearer. 
Further, decolorization by adsorption probably also involves 
the removal of the last traces of emulsified grease. It will 
be clear, therefore, that in the improvement in bright- 
ness and colour of a gelatine sol, adsorption fulfils a triple 



CLARIFICATION AND DECOLORIZATION 239 

usefulness. The ordinary processes of dyeing fabrics or leather 
are adsorption processes, and the decolorization of gelatine 
sols consists essentially of the same process, except that the 
concentration of the dyestuff is much less, and the liquor 
remaining, instead of the adsorbent, is the primary con- 
sideration. 

Decolorization of gelatine sols may be effected by any 
substance with a large specific surface (see p. 201). Indeed, 
a great variety of adsorbents are actually used in practice, 
and each factory has its favourite material or mixture, and 
its favourite mode, place, and time of application, determined 
partly by the nature of the adsorbent and partly by the 
precise form of apparatus used. Amongst the adsorbents 
which have received special favour are sand, kieselguhr, 
asbestos, animal charcoal, wood pulp fibre, albumin and 
alumina. Sand is very effective, but a comparatively large 
weight is needed, and its cleansing for repeated use is trouble- 
some. On the other hand, it may be completely renovated 
by ignition. Kieselguhr is a very powerful adsorbent, and 
only a little will do much good; it is, however, hardly 
sufficient alone. Animal charcoal has great specific surface, 
but its pores are very small for viscous liquors, and its use is 
less suitable in the case of gelatine than in the decolorization 
of liquors which may be boiled. Wood pulp fibre is a very 
popular decolorizing material, not only in gelatine but also 
in other trades. Its short, woolly fibres give a clarifying 
as well as a decolorizing effect. It may thus act as a 
mechanical filter for suspended matter and grease, as well as 
an adsorbent for colouring matters present as sols. Its two 
functions, however, are often confused. It may be re- 
generated for repeated use by careful washing, and special 
pulp-washing machines are manufactured and sold for the 
purpose. Detergents are usually employed in the wash 
waters. Asbestos is also a good adsorbent, and its long 
fibres make it much less liable to non-operating " channels " 
and " bursts." It also has the advantage that, if desired, 
it may be regenerated by ignition. It forms a very useful 
mixture with pulp fibre. 



240 ANIMAL PROTEINS 

All the above decolorizing materials are insoluble and 
hydrophobe, and act in virtue of their finely divided con- 
ditions, which causes them to have a large specific surface ; 
but there is another type or branch of substances, whose 
effect is due to surface action of rather a different type. 
These are the hydrophile gels. In a gelatine sol the colloid 
particles have largely adsorbed the colouring matters which 
it is desired to remove. This adsorption, which is after all 
only an equilibrium, is reduced by introducing another very 
strong adsorbent. This latter, by adsorption from the 
continuous phase, reduces the adsorption of colouring matters 
by the gelatine particles. In the case under discussion 
another lyophile colloid is introduced, and after bringing 
about such an action is removed by appropriate means. 
The use of albumin has long been known for such a purpose, 
its special advantage being that after its admixture and 
adsorptive action, it may easily be removed by raising the 
temperature above 70 C, when coagulation takes place, and 
b}^ subsequent mechanical filtration. The coagulated albumin 
takes down the adsorbed colouring matters. Albumin 
has been used in this way not only for gelatine and glue 
liquors, but also for tanning extracts (Part I.. Section III., 
p. 37) and other commercial preparations. Into this class 
of decolorizing agents fall the insoluble inorganic gels which 
have been advocated by W. Gordon Bennett, e.g. alumina 
cream. Freshly precipitated alumina hydrate is a colloid 
gel with very considerable adsorptive powers. It has also 
the advantage that it is quite insoluble, easily removed in 
filtration, and has a powerful adsorptive action upon other 
objectionable impurities, especially the poisonous metals, 
arsenic, copper, zinc and lead. Its use is an undoubted 
advantage when in addition to the other clarifying agents and 
adsorbents. It is conceivable, in some cases, that when alum 
is employed as clarifying agent in an alkaline gelatine liquor, 
some alumina may be formed, and as such contribute to 
the total effect. 



Section V.— BLEACHING 

The adsorption law indicates that however much colouring 
matter is removed from the volume concentration (continuous 
phase) there must always be some left. After all that the 
decolorization processes can do, there still remains much 
colour that can only be removed by a chemical action of the 
ordinary sense. The amount of colouring matter of this 
kind is not large, but it is a deep red-brown, and when the 
gelatine sol has been evaporated and dried out the final 
product, if untreated, possesses this typical colour, and is 
known as glue. If, before gelation, a chemical bleaching 
action is applied to destroy this pigment, the product may 
be then dried out in a nearly colourless condition and is 
known as gelatine. Gelatine, therefore, is simply bleached 
glue. Many other definitions have been given, and ma,ny 
elaborate distinctions drawn, but the fact of bleaching is 
the essential difference. In these days when gelatine is so 
valuable, the higher-grade products are nearly always 
bleached, and the term "glue" is consequently more often 
applied to a lower-grade product, and is sometimes used in 
a sense implying this fact. 

If it be desired to manufacture gelatine, it is fairly 
obvious that the task is lightened by observing the axiom 
that prevention is better than cure. If steps are taken to 
prevent the presence or development of such colouring 
matter, a great advantage is attained, for not only is the 
problem of bleaching easier, but also quicker and less expen- 
sive in chemicals. The nature of the colouring matters 
is but imperfectly investigated, but in the case of skin 
gelatine the pigment of the hair roots and epidermis is 

K, 16 



242 ANIMAL PROTEINS 

doubtless one factor. A long liming is said to assist in its 
destruction, possibly because this completes the loosening 
of epithelial structures and possibly because the alkali 
causes some hydrolysis of the pigment. In both skin and 
bone gelatine sols, however, there is a considerable tendency 
to develop the brown colouring matter typical of glue. 
This tendency is enhanced by an increase in temperature 
and also by the presence of acid or alkali. These facts seem 
to indicate that its development is associated with a partial 
hydrolysis of the gelatine in some direction. Rideal says 
this colouring matter is allied to caramel. In harmony 
with this is the experience that its development is greatest 
in products which have been ''burnt," i.e. subjected to 
unusually high temperature. The practical maxims which 
arise from these considerations are fairly obvious and widely 
known, viz. to conduct the extraction and evaporation at 
as low a temperature as possible and in as neutral a con- 
dition as practicable. The temperature is particularly 
important during evaporation (see Section VI., p. 249). 

Fortunately for manufacturers of gelatine, the colour- 
ing matter to be attacked is very susceptible both to re- 
duction and to oxidation, and both types of bleach are 
widely used in practice. It is somewhat curious that the 
same colouring matter should be destructible both by 
reduction and by oxidation, but there is no doubt that each 
type gives a perfectly satisfactory bleaching action and can 
result in a practically colourless gelatine. On the other 
hand, the reduction is the more unstable reaction, for the 
glue colour slowly develops again in the gelatine on keeping 
it, even in a dried condition. Gelatine bleached by oxida- 
tion, however, retains its colour quite well, and even tends 
to improve with keeping. It is quite possible that quite 
different reactions are involved in the two processes, but in 
the light of the above facts it is somewhat surprising to 
observe Rideal's statement that reduction followed by 
oxidation has been successful in practice. 

Although there is a wide choice of reducing and of 
oxidizing agents, those which are suitable for application to 



BLEACHING 243 

gelatine cover a very limited field. This limitation arises not 
so much from the ineffectiveness of the bleach, as from the 
other effects of these substances upon the purity of the pro- 
duct and upon the elasticity of the gel which it can yield. 
Especially important is the lyotrope influence of the bleaching 
agent. Many reactive substances are ruled out simply be- 
cause they either insolubilize the gelatine or weaken the gel 
it makes. Others are inadmissible on account of their 
poisonous nature. It must never be forgotten that whatever 
is used in bleaching is, like the gelatine itself, much con- 
centrated during evaporation and drying. Its possible 
percentage in the finished product should be considered, 
and also the possibility that in these finishing operations 
what is present may not remain in solution, owing to super- 
saturation. 

Bleaching by Reduction. — Of all the reducing agents 
suggested, sulphurous acid has proved to be much the 
most suitable and successful. It has been used with equal 
success both for bone and for skin gelatine, but on the whole 
has proved more suitable for the former. 

Sulphurous acid can fulfil in this instance a double 
function, viz. that of acid solvent for the bone phosphate, 
and that of bleaching agent also. As it penetrates the bone 
material, dissolving the phosphate, it also exercises its 
bleaching influence on the gelatinous part of the material. 
Changes of liquor tend to complete both actions, so that a 
counter-current system is found most convenient. The 
" acid process " for the manufacture of bone gelatine has 
been previously described (Section II., pp. 224-227), and 
the use of sulphurous acid in this connection is typified in 
the Bergmann process (p. 227). In this process bleaching 
is in effect merely a continued treatment. 

In the case of skin gelatine, also, sulphurous acid may 
fulfil a double function, viz. that of deliming agent as well 
as of bleaching agent. In such instance it is necessary to 
use excess of bleaching acid, some acting as deliming 
material and the remainder as bleaching agent. As it is 
desirable to get rid of the lime and soda salts, several 



244 ANIMAL PROTEINS 

changes of liquor are given to the goods, possibly with 
intermediate washing. Here again approximation to a 
counter-current system is of advantage, as the employment of 
used bleach liquors for deliming purposes effects considerable 
economy of sulphurous acid. Indeed, there need be no 
waste acid at all. 

Whether the material be for bone or skin gelatine, 
however, it will be seen that the extraction is conducted 
in an acid condition and the resulting sol is also acid. Most 
usually the decolorization and filtration processes are also 
conducted with such an acid sol. From what has been 
said (Section IV., p. 235) of the value of dibasic inorganic 
acids as clarifying agents, it will be understood that the 
presence of sulphurous acid at this stage is of great advantage 
in the production of a clear and bright gelatine. Indeed, 
it is well known in trade circles that sulphurous acid gelatines 
are usually of exceptional clarity and brightness. 

The disadvantage of sulphurous acid processes is also 
found in the same fact that both sol, gel and cake are in an 
acid condition. To complete the bleach it is sometimes 
necessary to add sulphurous acid to the sol after extraction, 
or even after evaporation, but this is to be avoided if possible. 
Usually the ideal attempted is that the bleaching action 
should be as much as possible before extraction ; the excess 
of sulphurous acid is then washed off just before the extrac- 
tion, as far as practicable, and the rest is boiled off during 
extraction. The ideal is practically never attained, for 
the acid is strongly adsorbed, and the result is that the 
finished article is always an acid gelatine, and sometimes 
indeed very decidedly such. The acid condition is objection- 
able in the case of some forms of filter press on account of 
the solvent action on the metals, and is objectionable in 
evaporation for similar reasons. Acid gelatines are also 
objectionable for many purposes for which gelatine is usually 
sold, and this limits the commercial possibilities of the 
product thus obtained. 

^ Sulphurous acid is itself, of course, a gas, and whilst 
the gas itself has been used for treating the material (e.g. 



BLEACHING 245 

bones), it has been found not only more convenient but also 
more effective to use an aqueous solution. This is mainly 
because it is possible to attain a greater adsorption in a 
liquor. Unfortunately, however, sulphurous acid is not a 
very soluble gas, and although 8-10 per cent, solutions may 
be, with great care, obtained, they are really supersaturated 
and readily yield the gas, even with slight mechanical 
agitation. Solutions even of 2 to 3 per cent, strength are 
also liable to this, and the general experience is that 1 to 2 
per cent, solutions are most economical and convenient for 
practical purposes. As the freight on weak solutions is 
prohibitive, the manufacturer using sulphurous acid is 
faced with the necessity either of purchasing cylinders of 
sulphur dioxide liquefied by pressure or making the gas 
and solution himself. The former is the most convenient 
course when only small amounts are required, but the 
latter preferable for a gelatine factory of any size. Sul- 
phurous acid is easily manufactured by burning sulphur 
and leading the fumes by induced draught up a scrubber 
down which water slowly trickles. Forced draught may 
also be used, as in the Sachsenburg plant. 

Of the other reducing agents which have been used, 
sodium hydrosulphite (Na 2 S0 2 ) deserves mention. It is 
a very powerful reducing agent, and has been found most 
useful when employed as an assistant to sulphurous acid. 
This reagent is usually added to the sol, after evaporation 
and before gelation. It is sold as a white powder, usually 
under trade names. Sometimes a mixture of bisulphite 
and powdered zinc replaces it, but this is objectionable for 
pure food gelatines. Its use also involves an impurity in 
the finished article, and a greater amount of " inorganic ash." 

Bleaching by Oxidation. — Many oxidizing agents 
have been suggested for bleaching gelatine, but most of 
them have some practical disadvantage. Most of them 
contradict the maxim (previously noted, pp. 222-223) that 
it is desirable to avoid adding any soluble substance, as this 
involves a permanent impurity, possibly concentrated to 
supersaturation in the finishing processes, and possibly 



246 ANIMAL PROTEINS 

involving a disadvantageous lyotrope influence. There is 
another objection to oxidizing agents also ; whilst their 
bleaching action on the pigments is undoubted, some of 
them have also a special action upon the gelatine itself 
which is in reality akin to tanning, and may indeed involve 
an insolubilization of the gelatine. Thus, chlorine gas 
(which Meunier patented for tanning) has been used for 
bleaching gelatine, but the conditions of success have not 
yet been thoroughly elucidated, and it is problematical 
indeed whether the process is consistent with best results. 
Hypochlorites and bleaching powder have also a similar 
action, which has been utilized with some success in 
practice. Rideal suggests that a suitable concentration for 
these reagents is i : 2000, and emphasizes the care necessary. 
An advantage of all these chlorinations is the formation of 
the strongly antiseptic chloramines, which preserve the 
gelatine from putrefaction. Ozone has also been tried as an 
oxidation bleach for gelatine, but not successfully, partly 
on account of difficulties in controlling the quantity used. 
Peroxide of soda has also been used, but it is not only 
alkaline, but liable to contain sodium hydrate and carbonate 
as impurities, and this involves neutralization either before 
use or in the gelatine sol, and the consequent presence of 
sodium salts in the finished article. Peroxide of calcium is 
open to the same objections, except that calcium is more 
easily removed from the sol than sodium. Rideal's suggestion 
for removing this lime, viz. precipitation by a current of 
carbonic acid, merits attention in this and in other directions 
also. Rideal also states that in the case of an acid bone 
gelatine, a good peroxide of lime is almost an ideal reagent 
for bleaching, inasmuch as " the lime carries down phosphate, 
several impurities and colouring matters." It thus acts 
as bleach, as neutralizing agent, and as precipitant, and the 
precipitate itself is a strong adsorbent. On account of its 
freedom from bases, and because its residue is simply water, 
peroxide of hydrogen has been found of great service in 
practice, and in most factories it has shown itself superior 
not only to the other peroxides, but also to all other oxidizing 



BLEACHING 247 

agents. Its application is simple, a concentrated solution 
being added to the gelatine sol before or after evaporation. 
It is the most " fool-proof " of all the oxidizing agents used 
in bleaching, and it yields the purest product. Its bleaching 
action is perfectly satisfactory, but only in a non-acid sol. 
Hydrogen peroxide is moderately stable in acid solution, and 
its bleaching action is best in slightly alkaline solution. 
An acid sol bleaches too slowly, or not at all ; an alkaline sol 
induces evolution of oxygen and consequent waste. The 
great disadvantage of peroxide of hydrogen is its great 
expense, which is enhanced by an increasing demand for 
it in other industries. A minor disadvantage is its instability, 
which leads to loss in transit and storage. It is sold usually 
in strengths indicated by the volume of oxygen obtained 
from unit volume of the solution, when treated with per- 
manganate in a nitrometer (e.g. " 15 vols, peroxide "). 

It is a fortunate feature of both the oxidizing and reducing 
agents usually employed in bleaching, that they have con- 
siderable antiseptic power. This assists materially in 
preserving the gelatine from putrefaction during the critical 
period between extraction and concentration. 



REFERENCES. 

" Glue and Glue Testing," S. Rideal, D.Sc, 2nd ed., pp. 61-66, 78-82. 
" Gelatine, Glue, and Allied Products," T. Lambert, pp. 29, 30, 49, 51. 
" Chemical Engineering," /. R. San. Inst., No. 2, 1910. S. Rideal. 
On adsorption phenomena : 

1. " Chemistry of Colloids," Dr. W. W. Taylor. 

2. " Chemistry of Colloids," V. Poschl. 

3. " Chemistry of Colloids," Zsigmondy and Spear. 

4. " Chemistry and Physics of Colloids," E. Halschek. 

5. " Surface Tension and Surface Energy," Willows and Hatschek. 



Section VI.— EVAPORATION 

The evaporation of the weak gelatine sols (3-9 per cent.) 
obtained by the processes described in previous sections 
into sols of such concentration (20-55 P er cent.) that they 
readily set to a stiff gel on cooling, is now an essential feature 
of gelatine manufacture, and is one of the most important 
processes. 

In the early days of this industry, manufacturers aimed 
at obtaining a concentrated sol, as this saved time in drying, 
and so reduced the possibilities of putrefaction. The advent 
of evaporation has reduced these possibilities to a minimum, 
and has also enormously reduced the space required and 
the capital outlay needed in the drying sheds. It has, in 
addition, given the practical advantages involved in dealing 
up to the last minute with a much less viscous liquor. As 
the liquors extracted are weaker, the extraction is more 
complete and the decolorization more easily effected. 

The earliest attempts at evaporation were not very 
successful, partly on account of the prolonged " stewing " 
which ruined the setting power, and partly because of the 
poor economy of heat. Thus in the open evaporators the 
sol was maintained at a high temperature for a long period, 
and this process only proved suitable for low-grade products. 

A great stride forward was made by Howard's invention 
of the Vacuum Pan. This made it possible to undertake 
concentration at much lower temperatures, a most important 
improvement in the case of gelatine and other organic 
matters easily damaged by heat. The process, however, 
was still slow, and the sol exposed to heat for a long time, 
as must be the case when evaporation takes place in bulk. 



EVAPORATION 249 

These disadvantages were still fatal to the production of 
the highest-grade gelatine. There were also the practical 
difficulties of entrainment ("blowing over"), in which 
parts of the sol were carried away by the escaping vapour, 
and also of " incrustation " which so rapidly reduces the 
heating efficiency and evaporative capacity of the machine. 
The vacuum pan, however, presented two decided advantages 
— evaporation at a low temperature, and, as a corollary, 
the possibility of utilizing exhaust steam to attain this 
temperature. 

Whilst the vacuum pan was a satisfactory machine for 
many branches of chemical engineering, the problem of 
evaporation was still unsolved for gelatine liquor because 
of the " stewing " involved, until the advent of the " film 
evaporator," which dealt with the liquor not in bulk, but 
in a continuous stream. In this way the product was only 
exposed to heat for a comparatively short time. Manj^ 
evaporators of this type came into being, and rapid improve- 
ment was made in the constructional details. The film 
evaporators retained usually the advantage of evaporation 
in vacuo, so that it was now possible to evaporate gelatine 
sols by exposure for a short time to a comparatively low 
temperature. Of this type of evaporator, the Lillie, Yaryan, 
Schwager, Claassen, Greiner, Blair Campbell, and the 
Kestner machines are well-known examples. 

A further advance in solving this problem was the 
application of the principle of multiple -effect evaporation. 
The vapour driven off during evaporation possesses of 
course many heat units, and is of very considerable volume. 
In multiple-effect evaporators this vapour is used to work 
a similar evaporator, and the evaporated liquor passes im- 
mediately into what is practically a second machine, and is 
further evaporated by the heat from the vapour just driven 
from it. Such an arrangement would be termed a double- 
effect evaporator. The vapour from the second effect 
may of course be similarly used to operate a third effect, 
and the vapour from this to work a fourth effect, and so on. 
Thus, we may have triple effect, quadruple effect, etc., even 



250 ANIMAL PROTEINS 

up to octuple effect. The great advantage of multiple-effect 
evaporation is in the saving of costly steam. Reavell gives 
the following figures to illustrate the economy thus obtained: — 

WATER EVAPORATED PER IOO UNITS STEAM. 



Single. 


Double. 


Triple. 


Quadruple. 


95 


I50 


220 


300 



There is naturally a limit beyond which the capital cost 
of the machine neutralizes the advantage of steam economy, 
and it is seldom that octuple effects are used. There are 
probably more triple effects in use than any other machine. 

An essential and important part of the modern evaporator 
is the " condenser," in which the vapour from the last 
effect is conducted into water (jet condensers) or over 
cooled surfaces (surface condensers), with a view to pro- 
ducing and maintaining the vacuum. 

A lasting vacuum cannot be maintained without an air- 
pump, as air is often introduced (1) with the steam, having 
entered the boiler dissolved in the feed water ; (2) b}^ leakage 
from the atmosphere into the condenser and the connected 
vacuous spaces; and (3) in jet condensers, in solution with 
the circulating condenser water. That from the first two 
sources may be reduced, but the third is beyond control : 
hence if high vacua are necessary, surface condensers are to 
be preferred. Dissolved air is usually 5-20 per cent, of the 
water volume, and is least for sea-water. It should be noted 
that water leaving a surface condenser is in a very air-free 
state, and therefore particularly suitable for boiler supply. 
Apart from the capital cost of a condenser the chief cost of 
maintaining a vacuum is in pumping the circulating water, 
of which up to 70 lbs. is usual per lb. of steam condensed. 

If W=weight of steam condensed (lbs. per hour) ; 

Q= weight of cooling water circulated (lbs. per hour) 
Ti=inlet temperature (° F.) of cooling water ; 
T =outlet temperature (° F.) of cooling water ; 
then 

To=T« + io 5 o(^) 



EVAPORATION 251 

It will be understood that for high vacua, low tempe- 
rature of cooling water (T<) is more important than copious 

supply (x^J- It is advantageous, however, to choose a 

site yielding plenty of cold water, such as a river or canal 
side. Otherwise it is often necessary to use cooling towers 
or spray nozzles. The cooling is by evaporation (=60 to 80 
per cent, of W), cold water replacing that evaporated, 

and yielding water 75 to 8o° F. If T 4 =8o° F. and S =70°, 

a vacuum of 28-34" is possible, but the 0-34" should be 
allowed for the partial pressure of the air, determined exactly 
by the air entering and by the displacement of the air-pump. 

Another feature of the modern evaporator is the " heater " 
or " calorifier," by which the liquor to be evaporated is led 
in acontinuous rapid stream through heated tubes immediately 
prior to its entry into the first effect. It is the aim of the 
heater to raise the temperature of the liquor to the tempera- 
ture of evaporation, and so to avoid this being necessary 
in the first effect. The heater thus further avoids stewing, 
ensures steady running, and effectively increases the capacity 
of a machine. 

It is noteworthy that superheated steam is not desirable 
for working an evaporator. The principle of evaporation 
by steam is not merely that the temperature of the liquor is 
raised to boiling point ; it is that in the condensation of the 
heating steam its latent heat is yielded to the liquor being 
evaporated. To evaporate quickly, therefore, the heating 
steam must condense rapidly. Hence, as superheated steam 
has a rate of condensation 20-30 times slower than saturated 
steam, the latter is much to be preferred. A slight super- 
heating, however, may be justifiable where the steam has 
any distance to travel before use. . It is the fact that it is 
the latent heat of steam which is mainly utilized which gives 
steam its great practical advantage over hot non-condensable 
gases. Steam in condensing yields an enormously greater 
number of heat units per lb. than hot waste gases. Steam 
has also the advantage of more constant temperature. 



252 ANIMAL PROTEINS 

The capacity and efficiency of an evaporator depends 
upon a good many factors, some of which are worthy of 
discussion at this point. 

The transference of heat and the amount of evaporation 
are directly proportional to the mean temperature difference 
between the heating steam and the liquor being evaporated. 
These temperatures, however, both vary somewhat, the steam 
losing part of its pressure and temperature as it passes along 
the heating surface ; the liquid generally increases in tempe- 
rature. The mean difference in temperature, moreover, 
is not the arithmetic mean between the smallest and largest 
temperature differences, but is given by the following 
expressions, which yield results not wide apart : — 
If 9 a ^temperature difference at commencement ; 
e = „ „ „ end ; 

and0 m =mean temperature difference ; 
then 

0,,,=^ or 
log£ 



<-V» 



This mean temperature difference is in practice usually 
spoken of as the " temperature head " or " heat drop." 
It will be clear that this temperature head is increased by 
using steam at higher pressure (temperature), and by 
evaporating under reduced pressure. Since most liquids 
have their boiling points reduced about 40 C. by operating 
in vacuo, the advantage of the vacuum is apparent. It 
should be remembered that the temperature head has not 
the same value in any part of the scale : it has more value 
higher up the scale, because the steam is denser and more 
heat units come in contact with a given area in a given time. 
It must also be remembered that whilst the pressure gauge 
is a most useful indicator of steam temperature, it is not 
necessarily accurate. The pressure in the hot space is the 
sum of the pressures of air and steam, and since the tempe- 
rature (the important condition) of the hot space depends 
upon the pressure* of the steam, and not on the sum of the 



EVAPORATION 253 

pressures, the temperature in a steam space is always rather 
lower than would be supposed from the pressure indicated 
by the gauge. 

The transference of heat is influenced by the velocity of 
both the heating fluid and the fluid being heated over the 
heating surface. The more rapidly each fluid moves, the 
more rapid is the transference of heat, because a greater 
number of particles of both fluids are brought to the heating 
surface in any given time. This is popularly known as 
the effect of " circulation," and is illustrated by the advantage 
of stirring a liquid being heated in bulk. In the film evapo- 
rators the circulation is through tubes at high speed (up to 
2 miles a minute), and the maximum effect in this sense is 
thus obtained. The increase in heat transference is not 
directly proportional to the increase in velocity, but in 
a lower ratio, sometimes approximately the square root of 
the velocity. In such a case, if either velocity be quad- 
rupled, the heat transference is doubled. Other advantages 
of high velocity are that the heating steam more readily 
sweeps away condensed steam from the heating surface, 
and the high-speed film similarly " scours " away " incrusta- 
tions " on the interior of the tubes. 

The transference of heat is also proportional to the con- 
ductivity of the metal forming the heating surface. For 
gelatine liquors, copper tubes are almost invariably employed, 
the advantage being great even when price is taken into 
consideration. The following conductivity coefficients illus- 
trate this point (calories per hour through 1 sq. metre of 
metal 1 metre thick, with a temperature difference of i° C.) : — 



Copper . . 


•• 330 


Tin 


•• 54 


Iron 


.. 56 


Zinc 


.. 105 


Steel 


22-40 


Iyead 


.. 28 



The coefficient of heat transmission decreases the more 
with increasing thickness of wall, the worse conductor is 
the metal. For copper tubes, however, this decrease is 
usually unimportant. 

The transference of heat is also much influenced by the 



254 ANIMAL PROTEINS 

viscosity of the liquor being evaporated ; the greater the 
viscosity, the lower the coefficient of heat transmission. 
Unfortunately for this process of evaporation, gelatine sols 
are exceedingly viscous, and thus the difficulty in obtain- 
ing a concentrated sol is thus greatly enhanced. 

The transference of heat is often greatly hindered by 
incrustations of the tubes, which incrustations generally 
conduct heat very badly. Thus the relative heat conduc- 
tivities of copper and chalk are as iooo : 5. 

The amount of heat transferred is of course determined 
also by the area of the heating surface. The amount of 
evaporation needed thus determines the number of tubes 
(of standard size) in the evaporator, and thus the capacity 
of the machine. An evaporator should have its heating 
surface area chosen with a view to the duty required of it. 

In practice the working of an evaporator is often not a 
very difficult matter, and large numbers of machines are 
operated by unskilled labour. Troubles generally arise 
from inconstant steam pressure, incrustation, leakages of 
air, which reduce the vacuum, the temperature head, and 
hinder heat transmission. For the evaporation of gelatine 
liquors the Yaryan, the Kestner, and the Blair-Campbell 
film evaporators are the most widely used. The velocity 
of the liquor through some of these machines is so great 
that occasionally no vacuum is used. The temperature 
obtained is high (200 F.), but the time is very short, if 
rapid cooling of the evaporated liquor is arranged. 



REFERENCES. 

" Evaporating, Condensing and Cooling Apparatus,'" by E. Haus- 
brand. Scott, Greenwood & Son (1916 Ed.). 

" Evaporation," by E. Kappeschaar. Norman Rodger (1914). 

" Evaporation in the Chemical Industry," by J. A. Reavell, M.I.Mech.E., 
J.S.C.I., 1918, April nth. 

" Glue and Glue Testing," S. Rideal, D.Sc, pp. 56-59. 

" Gelatine, Glue, and their Allied Products," T. Lambert, pp. 26-29. 

" Notes on Condensing Plant," J. M. Newton, B.Sc, /. Junior Inst. 
Engineers, Aug., 191 2. 



Section VII.— COOLING AND DRYING 

The conversion of a gelatine sol into cakes of gelatine has 
been much simplified by the advent of the evaporator. 
Before this machine was used much trouble was experienced 
with putrefaction, and in hot and thundery weather, especially 
on the Continent, it was often necessary to suspend opera- 
tions. Evaporation has, however, materially contributed 
to the possibility of rapid and satisfactory cooling and drying. 

From the time the weak sol is decolorized and bleached, 
the finishing processes consist essentially in the removal of 
water. This is now usually done partly by evaporation of 
the sol, and partly by the desiccation of the gel. There is 
an obvious elasticity in method, and factory practice does 
actually vary considerably in the relative proportions of 
these two alternatives. Some factories evaporate to a 
20 per cent, sol, approximately, and rely upon drying 
sheds and lofts to complete the desiccation : other factories 
evaporate up to a 55 per cent, gelatine sol, and so can manage 
with less shed room. Something depends upon local con- 
ditions, but the main issue is between the cost of steam in 
evaporation and the cost of land and buildings required for 
sheds. On the whole the modern tendency is to evaporate 
more, for this course has the additional advantage of speed, 
involving both a quicker turnover and less liability of 
putrefaction. Lower-grade products need relatively greater 
evaporation to form a gel of equal rigidity. 

After evaporation and bleaching, the concentrated sol 
is first cooled rapidly until it has set to a stiff gel, then cut 
up into cakes according to the size required, these being dried 



256 ANIMAL PROTEINS 

out on network frames arranged in tiers, through which a 
draught of air is usually forced or induced. This general 
description is of course applicable to many factories with 
innumerable variations in detail, most of which variations 
originate in local convenience and are unessential parts of 
the manufacture. 

An essential principle is that the cooling or gelation 
should be done rapidly, not only to avoid putrefaction but 
also to avoid the action of heat on the elasticity of the gel. 
A hot sol or gel is liable to hydrolysis and loss of setting 
power, and should have its temperature quickly reduced, 
but a warm sol or gel (say ioo° F.) is most liable to putre- 
faction, so that the cooling should be continued quickly. 
On the other hand, the gel should not be frozen. For cooling 
purposes a copious supply of cold water is most usually 
employed, but some factories have installed refrigerators. 
These plants operate by the rapid evaporation of liquefied 
gases such as carbon dioxide, sulphur dioxide, or ammonia, 
so arranged as to cool a solution of common salt, which 
forms the circulating liquor and is returned after use to the 
' refrigerator. Where such plants are used, it is natural that 
their use should be extended to the drying sheds to cool the 
air entering in the height of summer. In some factories 
the cooling is attained neither by cold water nor cooled brine, 
but merely by cold air. 

The kind of vessel in which gelation is induced varies 
widely in different factories. For lower-grade products 
metal boxes are used, heavily galvanized iron being the 
most common material. If the liquor be muddy, deep 
boxes are preferred, but if clear, rapid cooling is best attained 
by having them long and shallow, and so exposing a relatively 
greater area to the cooling action. In either case the boxes 
may contain up to \ cwt. of jelly. Lambert mentions 
boxes 24" X 6", which are 5" deep ; Cavalier suggests rect- 
angular moulds holding 30 litres. In place of galvanized 
sheet iron, boxes of sheet zinc or of wood lined with zinc 
are sometimes used. In any case the most scrupulous 
cleanliness should be observed in all cooling-house work, 



COOLING AND DRYING 25? 

and in some factories the most elaborate precautions are 
taken for cleansing vessels, tools, floors, etc., and even for 
their disinfection and sterilization. Iron, tinned iron, and 
copper cooling vessels are ruled out on account of their 
tendency to rust and tarnish, and the last is unjustifiably 
expensive. Many of these vessels are unsuitable for pure 
food gelatines in which traces of copper, zinc and arsenic 
are held to be very objectionable. For the best gelatines, 
therefore, a very shallow vessel {\" to J" deep) with a sheet 
glass bottom is preferred, and the concentrated sol is run on 
to this for gelation. 

Glue (or gelatine) which has set in this way is some- 
times called " cast glue." That which sets in metal boxes 
in blocks is termed " cut glue," because the blocks of jelly 
need subsequently to be cut into slabs of the desired size 
and shape. Jelly blocks may be cut by hand with the 
" wire knife " which yields a characteristic wavy appear- 
ance to the finished product. This may also be done by 
machinery, the block of gel being placed on a series of 
correctly spaced wires and forced through the network by 
hydraulic pressure. A cutting machine (Schneible) has also 
been used to cut up blocks of jelly into slices of the required 
thickness, but these machines have not made great headway 
in this country. It will be clear that cast glue is cooled more 
rapidly than glue in blocks ; it is therefore not surprising 
to note Iyambert's statement that the former comprises the 
larger proportion on the market. 

The cut or cast cakes are next placed upon network 
frames, and a series of such frames are placed on a bogey. 
The bogey is run along tram lines into the drying tunnel, 
through which air is forced or induced by a fan. Many such 
bogeys are, of course, passed into each tunnel, and as many 
tunnels as required may be constructed. Care is necessary 
to expose the cakes evenly to the action of the air. It is 
mostly necessary to warm the air at the inlet by means of 
steam pipes and so increase its drying power. This is 
especially necessary in winter or wet weather. In summer, 
however, it is often arranged that the air is cooled before 

E. 17 



25S ANIMAL PROTEINS 

entering the sheds. This is accomplished by passing the 
air through pipes from a refrigerator. When heated air 
is used, it is stated by Lambert that the maximum tempe- 
rature should be 25-5° C. (78 F.) ; Rideal considers 21 C. 
(70 ° F.) should be the maximum. In all cases the drying 
power of the air is easily ascertained from a wet-and-dry bulb 
thermometer, and the amount of air passing along the shed 
from a wind gauge. Lambert states that drying normally 
occupies four to five days. The final product is still a gel, 
of course, and contains from 10 to 18 per cent, of water. It 
appears, however, very hard and solid. The dried cakes are 
removed from the frames and transferred to the warehouse, 
where they are sorted according to quality and packed in bags 
or tin-lined boxes. Some material is ground to powder. 

The network of the drying frames has been made from 
many materials. Cotton or string netting is very common, 
but is liable to sag and to get dirty. It also has a short 
life. Ordinary galvanized iron soon loses its galvanizing 
cover, and the iron then is liable to rust. Attempts have 
been made to use sheet zinc and other alloys, which are 
cut or punched into nets with square or diamond-shaped 
holes. These were found to warp and break. Rideal's 
conclusion, which is confirmed by the general experience, 
is that the best material is a heavily galvanized iron wire 
netting. He suggests that it should have 15 to 25 per cent, 
of its weight of zinc, and that it should be strengthened by 
stiff er ribs arranged both longitudinally and transversely. 

Many attempts have been made, and many patents 
taken out, with the object of making the cooling, cutting, and 
drying processes as continuous and as quick as possible, and 
with a view to saving labour, which is rather costly at this 
stage. These attempts, however, have only met with 
indifferent success. A common idea is that a continuous 
supply should fall upon a revolving appliance, and be 
instantly congealed in a thin state, which last lends itself 
to more rapid desiccation. Vacuum drying has also been 
attempted. 



COOLING AND DRYING 259 



REFERENCES. 

" Glue and Glue Testing," S. Rideal, D.Sc, pp. 68-74. 

" Glue, Gelatine, and Allied Products," T. Lambert, pn 10-35 

Chem. Zeit., 1911, 35, 17 (Cavalier). 



PATENTS. 

Eng. Patent (1894) 11,426 (Hewitt). 

Eng. Patent (1898) 2,400 (Brauer). 

Fr. Patent (1909) 398,598 (Lehmann), J.S.C.I., 1909, 897. 

U.S. Patent (1912) 1,047,165 (American Glue Co.). 



Section VIII.- USES OF GELATINE AND 

GLUE 

Gelatine and glue have both been put to an immense 
variety of uses, and the list is constantly extending. Indeed, 
no one who considers the following account of their applica- 
tions can doubt that gelatine and glue have become a neces- 
sary part of our civilization. 

Gelatine for edible purposes certainly forms a very 
considerable part of the total used, and great pains are now 
taken to obtain a pure product. Thus, a gelatine with more 
than i *4 parts per million of arsenic, or more than 30 parts 
per million of copper, is not considered good enough for 
"pure food." The food value of gelatine, compared with 
other proteids, is exceedingly low ; its use in this connection 
has no connection with the " calories " of heat energy it 
will yield. It is used almost entirely because of its property 
of forming a gel. Table jellies form, of course, one popular 
use of gelatin, but the manufacture of sweets makes also 
a great demand upon the gelatine trade. Culinary opera- 
tions often require a little gelatine, especially is it used in 
pies and soups. An extension of the same idea is found 
in its employment for many manufactured foods, e.g. tinned 
meats, meat extracts, and the concentrated foods. The use 
of gelatine in connection with the first of these received a big 
impetus during the war period. In gelatine for any of these 
purposes, the presence of excess of sulphurous acid is 
objectionable, as its taste is easily noticed. 

Gelatine for medicinal purposes finds an ever-growing 
number of applications. Gelatine capsules for holding 
greasy liquids and solutions of nauseous drugs are in- 
creasingly popular, for the dose may be swallowed without 



USES OF GELATINE AND GLUE 261 

unpleasantness. In making these capsules some sugar is 
also used, and the finished article is often protected from 
atmospheric moisture by treatment with a weak solution 
of alum. In a similar way pills are often coated with a 
33 per cent, gelatine sol. Such pills are not only pleasanter 
to swallow, but are less liable, after being dried, to stick 
together in the box. Acohol solutions of drugs (or essences, 
perfumes, etc.) may be suitably stored in gelatine instead 
of metal tubes. Medicated wines are detannated b}^ 
gelatine before the addition of drugs which would have 
been precipitated by the tannin. The British Pharma- 
copoeia specifies four kinds of "Iyamellse," which are 
small discs of gelatin and glycerin, each containing a minute 
but definite dose of some powerful alkaloid. Glycerin 
jelly is a mixture of gelatin glycerin with some water. 
It is used for chapped and rough hands ; the mixture is 
also used for glycerin suppositories, and for mounting micro- 
scopic sections. The mixture also forms the basis of gelato- 
glycerin, used in nasal bougies, and of glyco-gelatin for 
medicated lozenges. Gelatine insolubilised by formalin 
(formo-gelatin) has been used for making tabloids, wound 
dressings, and artificial silk. 

Gelatine is in constant demand for bacteriological work, 
for which purpose a high-grade product is desired. Nutrient 
media for the culture of bacteria are solidified by 10-15 per 
cent, of gelatin, and the growth of colonies of bacteria often 
show typical formations. By inoculating into a melted 
and sterile quantity and setting quickly in a flat dish after 
mixing, the number of bacteria in the volume introduced 
can be judged from the number of colonies which develop. 
Bacteria are also distinguished often as " liquefying " or 
" non-liquefying " according to their type of culture on 
nutrient gelatine media. Gelatine for such work should be 
neutral and of high clarity. 

The gelatine required for photographic purposes is also 
a high-class product. It should be neutral, colourless, and 
free from chlorides and other mineral salts. Grease also is 
objectionable. Gelatine is used in the numerous carbon 



262 ANIMAL PROTEINS 

processes, in which the principle is that gelatine is made 
insoluble in water by the action of potassium dichromate 
under the action of light. It is used also in Poiteoin process 
for copying engineering drawings, which is based upon the 
power of a ferric salt to render gelatine insoluble so long 
as it is not exposed to the actinic rays. 

Gelatine is used in the manufacture of the " crystalline 
glass " used for decorative purposes. Advantage is taken 
of the immense contractile force it exerts on drying. When 
ground glass is coated with gelatine, and the latter dried, it 
tears away the surface of the glass itself, and leaves peculiar 
fern-like patterns. Inorganic salts dissolved in the sol 
influence the nature of the pattern obtained. 

Gelatine is used also very largely in the textile trades, 
for finishing coloured yarns and threads, for sizing woollen 
and worsted warps, and for thickening the dyestuffs used 
in printing fabrics. It is also used for finishing white straw 
hats ; as a size in the manufacture of high-class papers, 
and as a wax substitute for covering corks and bottle necks. 

Glue is used instead of gelatine in all cases where colour 
is not a matter of much moment. The fact that it has not 
been bleached makes no difference to its suitability in such 
a case, and the cost is substantially reduced. Thus, for 
dark-coloured straw hats, textiles, sweets, papers, and in 
all suitable woolwork, glue is used in place of the more 
expensive article. 

A very large quantity of glue is used in the manufacture 
of matches, where it functions as the material binding the 
" head " to the stem. A 15-50 per cent, sol is used, con- 
taining nitrate or chlorate of potash as oxidizing agent. 
The mixture is kept at 38 C. and the phosphorus cautiously 
added, and when this is emulsified, the friction ingredients 
(sand, glass, etc.) are also added. The glue acts also in 
preventing premature oxidation. Glue is also used in 
making the match-boxes, and similarly in making sand, 
emery, and glass papers and cloths. 

There is a large consumption of glue by joiners, carpenters, 
cabinet-makers, and all kinds of woodwork and fancy work. 



USES OF GELATINE AND GLUE 263 

It is used in the manufacture of furniture of all kinds, of 
pianos, organs, billiard tables, panels, picture frames, and 
of toys and brushes. Mixed with white lead, chalk, and 
sawdust, it forms a composition used for mirror frames, 
rosettes, etc. Glue is used for veneering, for mosaics, 
plaques, trays, fingerplates, leather wall converings, and 
for staining floors. 

There is also a considerable sale for glue in book-binding, 
for which a sweet, light-coloured, and strong product is 
required. It has been found particularly suitable for 
leather bindings where the grain has been artificially printed 
or embossed (see pp. 97 and 117), and in finishing and gilding. 

The compositions used for printing rollers all contain 
gelatine or glue together with sugar or glycerin and possibly 
oil and soap. They are often hardened with formalin. 
Similar mixtures are used for the beds of hectographs. 

Glue (together with waste leather) is used in the manu- 
facture of imitation leather and leather substitutes. Cotton 
and wool fibres are often incorporated, and sometimes 
textile fabrics. 

Much glue is coverted into " size," which is a weak gel 
used as a filling rather than as an adhesive agent. A low- 
grade glue is often therefore preferred for such purposes, 
as having " body " rather than " strength." Size is often 
sold in cake, but sometimes in the form of the gel itself, 
in which case it may never have been evaporated. Indeed, 
size is often overboiled glue, made by crude and out-of-date 
methods. It is largely used in the paper trade, and for wall- 
papers, millboards, papier-mache, paper and cardboard 
boxes, etc. Mixed with logwood and iron, and possibly 
alum, it formed the " blue size " once largely used by boot- 
makers as a foundation for blacking, and is similarly used 
in currying (p. 82). Size is also used in making oil paints 
and varnishes. Distemper is a size with which is incorporated 
whiting or gypsum and coloured pigments. In all applica- 
tions of size, it is common to use antiseptics. Salicylic 
acid has been widely used in this sense. Low-grade glue is 
used for the manufacture of cheap brushes and for fly-papers. 



264 ANIMAL PROTEINS 

Innumerable patents have been taken out and mixtures 
invented for the production of plastic materials, which 
frequently involve gelatine or glue. Thus, gelatine and 
glue are used in making plaster casts, and for imitation 
ivory, wood, stone, and rubber. Many of these inventions 
have been investigated by Rideal, who points out the features 
common to most of them. Usually a viscous sol is thickened 
by the addition of inert fibres and powders, and with the 
object of making the preparation more waterproof it is 
customary to incorporate oils, fats, waxes, tars, and resins 
before the gel is set. The surface is hardened b}'' " tanning " 
with formalin or tannin solution, finally painted or varnished. 

Equally innumerable are the inventions, recipes, and 
patents for making glues that shall remain liquid. The 
convenience of this ideal is obvious, but many of the sug- 
gestions are useless. It is quite easy to incorporate into a 
gel substances which keep it liquid — any soluble substances 
with a lyotrope influence of the iodide type will do this — 
but these also prevent the glue setting when used. Even 
in small quantity they will influence the tenacity of the 
joint. Other methods depend upon a partial hydrolysis 
of the protein. Amongst the most successful of these 
attempts are to dissolve 3 parts of glue either in 12-15 parts 
saccharate of lime, or in 9 parts of 33 per cent, acetic acid. 

Many special glues and cements are made from com- 
mercial glue, according to the purpose required. " Marine 
glue " contains no glue ; it is made from shellac and rubber 
mixed with benzene or naphtha. Its advantage is water- 
proofness. 



REFERENCES. 

" Glue and Glue Testing," S. Rideal, D.Sc, 2nd ed. 

" Uses of Glue," chap. iii. p. 83. 

" Uses of Gelatine," chap. iv. p. 100. 

" Special Glues," p. 108. 

" Liquid Glues," p. 119. 

" Gelatine, Glue, and their Allied Products," T. Lambert. 

" Uses of Glue and Gelatine," chap. ix. p. 80. 

" Liquid Glues and Cements," chap. viii. p. 69. 



Section IX.— THE EVOLUTION OF THE 
GELATINE AND GLUE INDUSTRY 

Thk manufacture of gelatine and allied products has received 
a great stimulus in this country from the circumstances 
arising from the European War. The large restriction of 
continental — especially French and Belgian — supplies of gela- 
tine, led to greater demands for the British-made product, 
and resulted not merely in a period of greater prosperity, 
but in a period in which much greater efforts were made to 
supply a high-grade article in larger quantities. Most 
manufacturers strove to make high-class gelatine rather than 
low-grade glue, great extensions were made, and many new 
businesses were established. The development of the leather 
trades, more particularly in respect of greater production, 
caused a bigger supply of raw material for skin gelatine, 
and the slaughter of home animals for food caused a more 
plentiful supply of bones. At the same time it was realized 
that greater production not only reduced working costs, 
but also that a bigger turnover in any one factory involved 
a proportionately less capital outlay. These facts tend 
to counterbalance the heavy freight on the raw materials. 
Production is thus not only on a larger scale but more 
intensive. 

One of the greatest difficulties of this industry is to 
produce a regular or standard article, for the raw material 
is so exceedingly variable in quality ; that for skin gelatine 
tends also to become less valuable. In such a case, as 
Rideal has truly remarked, to ensure that supplies to customers 
shall be always " up to sample," which is often a matter 



266 ANIMAL PROTEINS 

of contract — " exact and regular working, strict cleanliness, 
observance of temperatures and other physical data, and 
scientific supervision, are clearly necessary. " Rule of 
thumb " is never quite certain to produce the same article 
twice. In past years British methods of manufacture 
have been far too empirical. As in other industries, " rule 
of thumb " must inevitably be replaced by scientific prin- 
ciple. The advances in colloid chemistry of this last 
decade or so have, in the author's opinion, supplied the clue 
to this line of development. In the preceding pages em- 
phasis as been laid upon the importance of the adsorption 
law, the lyotrope series, and the valency rule. The manu- 
facturer or supervisor who understands and can apply 
these generalizations will find his task vastly easier and 
his factory more efficient. Much remains to be learnt, 
however, and the industry would certainly benefit by 
research work, for which there is a fertile field. 

There is also considerable room for improvement in the 
methods of chemical engineering usually employed. Whilst 
the heat engineers have certainly done much to solve the 
question of evaporation and drying, there is still great scope 
in the more economical application of heat in extraction, 
and the last word can hardly have been said on the problem 
of clarification and decolorization. There is indeed almost 
as much scope for research by the chemical engineer as by 
the colloid chemist. 

The industry also exhibits, in common with the leather 
and many other trades, the same tendency to save labour, 
both by careful arrangement of the factory and by the 
installing of mechanical labour-saving devices. Thus, lifts, 
runaways, hoists, trucks are increasingly used to move the 
solids, and pipes and pumps to move the liquors. As ever, 
there is scope for the mechanical engineer. 

If some of these problems are vigorously tackled duiing 
the present reconstruction period, there is little doubt that 
the gelatine and glue industry will be in a much better 
position to cope with all possible competition in the future. 

From what has been said in Section VIII. as to the wide 



EVOLUTION OF THE GELATINE INDUSTRY 267 

uses of gelatine and glue, it will be seen that general 
prosperity in trade is conducive to better trade conditions 
in the gelatine and glue industry. It is similarly true that 
a general trade slump affects the glue trade adversely. The 
severe trade depression which commenced in 1920 has had 
this effect, and has made economic produdion much more 
difficult as well as more essential. As often is the case, the 
larger factories and firms can better face the difficulties, and 
there can be little doubt that if the depression be long 
continued there will be a tendency for the smaller factories 
to be closed down and for the larger firms to unite. As in 
the leather trade, both the War boom and the Peace slump 
have caused the gelatine and glue trade to develop along the 
lines of the great trusts. It may be reasonably expected, 
moreover, that these will be intimately connected with the 
leather trusts. This fact, together with the heavy freight 
charges on the raw material, tends also to make the skin 
glue factories gravitate towards the leather centres. 



Part VI.— MISCELLANEOUS PROTEINS 
AND BYE-PRODUCTS 

Section I.— BYE-PRODUCTS OF THE 
LEATHER TRADES 

In the leather trades by far the most important and valuable 
bye-products are obtained from the hides and skins them- 
selves, and all these are obtained before the tannage proper 
is commenced. The leather trades use only the dermis 
(corium) or true skin for the manufacture of leather, and as 
we have noted (Part I., Section II., p. 16) this prepared and 
purified dermis is called " pelt." The cuttings and trim- 
mings from the pelt form the most valuable bye-product of 
the leather trades, and are the raw material of the gelatine 
and glue industries (Part V., Section II., p. 221). Many 
portions of the pelt, indeed, such as ears, noses, and cows' 
udders, are quite useless for any other purposes. Other 
portions, such as cheeks, faces, and even bellies, may be 
made either into glue or leather according to the state of 
trade. Hardly less important to the same industry are the 
cuttings of adipose tissue removed in " fleshing " the hides 
and skins. These, though yielding less protein, yield also, 
however, the valuable animal greases (Part V., Section II., 
p. 227). To obtain both these products in a purer condition 
the removal of " flesh " after " soaking," but before 
" liming " (Part I., Section II., p. 18), has been favoured by 
some, especially in America. 

Amongst the epithelial structures of the hides and skins, 
we have several protein bye-products which have some 



BYE-PRODUCTS OF THE LEATHER TRADES 269 

commercial value. The horns of cattle are now almost 
invariably removed before reaching the leather manu- 
facturer, but have some little value. This part of the 
epidermis is not solid keratin. A " pith " is easily removed 
after boiling in water. The outer parts, too, are often 
coarse and somewhat damaged, but if removed by scraping 
reveal often a rather beautiful structure of varying colour. 
There is some opening for this product in the manufacture 
of small articles of horn, but much of it, together with 
hoofs, is roasted and crushed for making fertilizers. The 
- hair of cattle, goat, etc., has also a commercial value. This 
is removed after liming, and needs subsequent purification 
(Part I., Section II., p. 22). The hair is well washed with 
water, using either repeated changes or a continuous supply, 
the operation being carried out in paddles or similar 
machines which stir up the hair in the water. When clean, 
the hair is transferred to a centrifugal machine or " spinner," 
in which much adhering water is removed. This is a great 
assistance in drying out, which is the next and final operation. 
In drying, the hair is laid upon steam-heated boxes or pipes, 
and a current of warmed air passed over or through it by 
means of a fan. It is better to have the hair " turned " 
occasionally. This ensures quicker as well as more even 
drying. The product is made up into large bales and sold 
for the manufacture of felts, mattresses, etc. White hair 
is usually kept separate and commands a larger price. 
The power consumed in driving the washing machinery, 
the centrifuges and the drying fan, together with the fuel 
required for the drying steam, and the labour involved 
throughout, make it doubtful whether this bye-product is 
worth either the capital outlay or the working costs neces- 
sitated. Many manufacturers avoid this treatment alto- 
gether, therefore, and the wet limed hair is sold direct to 
the fertilizer factory. A less price is obtained, but much 
expense is saved. Especially when the animals have only 
their short summer coats, this course is preferred. 

In the case of the wool from sheepskins the product 
is much more valuable. The wool, indeed, is often the 



270 ANIMAL PROTEINS 

primary consideration. Unfortunately this sometimes 
results in the neglect of the pelt. The removal of wool 
from sheepskins forms a special industry known as " fell- 
mongering," which has been previously described (Part II., 
Section IV., p. no). Pains are taken to clean the wool 
even before removal from the pelt, by the liberal use of 
water and the " burring machine." There is much variation 
in quality, and care is taken to keep the various grades 
separate, even during the " pulling " operation. From the 
fellmonger the wool passes to the " wool stapler," and 
forms the basis of one of our most important mechanical 
industries, the manufacture of woollen cloths. Wool is also 
removed from sheep by the periodic shearing, and in this 
case does not reach the fellmonger at all. 

Apart from the raw material itself, there are few bye- 
products of the leather trades which are of commercial 
importance. The sludge from the pits of the limeyard 
contains, in addition to much lime and chalk, a certain 
proportion of protein matter. This is derived partly from 
the blood and dung associated with the hide, partly from 
the solution of the corium hide substance, partly from the 
solution of the softer keratins, and partly also undissolved 
and loose hair. This bye-product is rather difficult to 
deal with, as it will not easily dry. It is indeed sometimes 
a problem to dispose of it, except in rural districts, where 
the farmers appreciate its manurial value and will usually 
cart it away for a nominal fee. Where possible, it is better 
to let it drain and settle on land, and pile it up in heaps to 
dry further. Soak-pit sludge has a distinctly greater value 
as manure, on account of the greater proportion of dung 
proteins. As some lime is often used in these pits, the 
product is a really useful fertilizer. 

The only other bye-product of the leather trades is 
waste leather itself. For small pieces of leather there is 
always some little opening in producing small articles, such 
as washers for taps, etc., and there is also the possibility of 
shredding or pulping and making an artificial leather. The 
best leather substitutes, indeed, are made from waste 



BYE-PRODUCTS OF THE LEATHER TRADES 271 

leather. Nevertheless, there is always a certain amount of 
waste leather which only finds an outlet in the fertilizer 
factory. Such material is usually steamed or roasted to 
make it brittle, and then crushed in a disintegrator. It is 
then mixed in with other materials, but is sometimes 
solubilized by the action of sulphuric acid. Leather seldom 
contains less than 30 per cent, protein. 



REFERENCES. 

"Chemical Fertilizers and Parasiticides," S. H. Collins, M.S., F.I.C. 
(Companion volume in this series on Industrial Chemistry.) 
" Wool Wastes," Part II., Section V., p. 75. 
" Hoofs, Horns, Leather," Part III., Section II., p. 115. 
" Gelatine, Glue, and Allied Products," T. Lambert. 



Section II.— BYE-PEODUCTS OF THE 
GELATINE AND GLUE TRADES 

From the skin gelatine and glue trades the most valuable 
bye-product is the grease, which is obtained from the 
" fleshings " of the adipose tissue. These fleshings are 
themselves a bye-product of the leather trades. The 
recovery and purification of this grease has been dealt with 
previously (Part V., Section II., p. 227). In the skin glue 
trade the only other bye-product is the residue from the 
extraction process (Part V., Section III., p. 230). This 
residue is known usually as glue " scutch," and is composed 
of the proteins of the skin which are insoluble in hot water. 
These insoluble portions are obtained from all layers of the 
skin. There is much hair often in scutch, the hyaline or 
glassy layer (grain), and the elastic fibres of the corium are 
also insoluble, and a proportion is derived from the fibres 
of the adipose tissue on the flesh side. All these portions 
are fairly rich in nitrogen, and the scutch has, therefore, 
considerable value to makers of fertilizers. It is liable to 
contain also a percentage of grease, which is usually removed 
by steaming under hydraulic pressure. This process recovers 
a valuable bye-product and increases the manurial value of 
the scutch. There is always left in scutch some of the 
gelatinous skin substance which, strictly speaking, should 
have been removed during extraction. There is, however, 
a practical limit beyond which it does not pay to do this. 
When this limit is reached the cost of steam in extracting, 
and also in evaporating and drying, together with the loss 
of time and labour involved by occupation of the plant, is 
greater than the value of the possible product. 

From the bone-glue industry, the grease is similarly a 



BYE-PRODUCTS OF GELATINE AND GLUE 273 

valuable bye-product, but there is also another of equal 
importance, viz. the phosphate of lime, which comprises 
about half the raw material. As previously described, in 
Part IV., Section II., p. 225, this is usually extracted after 
the grease, by solution in weak hydrochloric acid. The 
solution is neutralized in lead-lined vats with milk of lime, 
a precipitate of di- and tri-calcium phosphates being obtained. 
Calcium chloride is left in solution, and the precipitate should 
be, therefore, well washed if it be desired to have dry 
phosphate. The bone-glue industry is, generally speaking, 
much more intimately connected with the fertilizer trades 
than the skin-glue trades, indeed the extraction of the bones 
for glue is not always advisable, in which case the protein 
matter as well as the phosphatic matter of the bones are 
employed for making " bone manures." For details of this 
industry the reader is referred to a companion volume in 
this series on " Chemical Fertilizers." 



REFERENCES. 

" Chemical Fertilizers and Parasiticides," S. H. Collins, M.Sc. 

" Bones," Part II., Section V., p. 72. 

" Precipitated Bone Phosphate," Part III., Section III., p. 157. 

" Bone Manures," Part III., Section V., p. 173. 

" Gelatine, Glue, and Allied Products," T. Lambert. 



Section III.— FOOD PROTEINS 

Although there are those who consider that animal proteins 
are both undesirable and unnecessary as foods, it is never- 
theless true that man is almost universally a carnivorous 
animal. The animal world provides mankind with one of 
its chief sources of food, and especially of protein foods. 
Protein foods are unquestionably essential, and animal 
protein foods differ chiefly from those of vegetable origin 
in the fact that they contain generally much more protein. 
Of the proteins noted in our Introduction, the keratins have 
no value as foods ; the gelatins have some value as culinary 
material, but little actual food value ; whilst the albumins 
comprise practically all the useful animal food proteins. 
Whilst the actual flesh of animals is the principal source of 
food proteins — both as to quantity and food value — other 
parts of animals, e.g. kidneys, liver, blood, brains, tongue, 
are used and relished. The most important sources of 
animal food proteins are from fish, fowl, sheep, cattle, and 
pigs, the meat from these being roughly in the same sequence 
as to digestibility. There are, however, many other animals 
of which the flesh is quite edible, but most of the above are 
specially farmed and propagated primarily for their food 
value. 

As the animal food proteins are exceedingly putrescible, 
they are usually consumed within a short time of the animal 
being killed. It is perhaps natural, therefore, that many 
efforts have been made to discover means of preserving 
such foods. These efforts form the basis of some important 
industries, and though they can hardly be included as 
chemical industries, it will not be out of place in this volume 
to point out that these efforts present analogies with, as 



FOOD PROTEINS 275 

well as differences from the methods used for preserving 
hides and skins (Part I., Section I., p. 12). The curing 
of hides and skins is a temporary preservation from putre- 
faction until the opportunity is convenient for the permanent 
preservation (i.e. tannage). The preservation of meats is 
analogous to curing inasmuch as more drastic treatment 
might indeed make them non-putrescible, but would also 
render them indigestible and unsuitable for food. Thus 
drying, salting, drying and salting, pickling and freezing, are 
just as suitable for preserving food proteins as for hide and 
skin proteins. Hence we have dried meats, salt bacon, 
pickled beef, frozen mutton, etc. To a limited extent 
smoking (fish, bacon, etc.) has been employed as a cure. 
When it has been applied to skins it is usually combined 
with a fat tannage. There is, however, one method of 
preservation of proteins, inapplicable to skins, which has 
been eminently successful and useful for food proteins, 
viz. sterilization by boiling. The food has been placed in 
tins, hermetically sealed, and thoroughly sterilized. Hence 
have appeared corned beef, tinned tongue, sardines, etc., 
which merely illustrate the immense possibilities involved. 
A noteworthy advantage of this method of preserving 
animal food proteins, is that the food is already cooked and 
prepared for immediate consumption. 

Another line of effort is the preparation of concentrated 
foods. Just as animal foods are on the whole more concen- 
trated in protein than vegetable foods, so these prepared 
animal foods are more concentrated than animal flesh, and 
generally also more soluble. Such preparations of animal 
protein are obviously useful when there is difficulty in 
swallowing and when journeys are necessary into regions 
of poor food supply. It is a little doubtful, one must say, 
whether the concentration is as great in some cases as is 
claimed. 

Yet another industry based upon the animal proteins is 
the manufacture of meat-extracts. These are not merely 
concentrated extracts of animal flesh, but contain especially 
the stimulative properties of animal food proteins. There 



276 ANIMAL PROTEINS 

is now little doubt of the value of these preparations as 
stimulants, and it is claimed for them that they not only 
have food value, but also that they increase the food value 
of other foods used with them. Together with these products 
may be classed all the miscellaneous tonic foods, in which 
proteins are blended with carbohydrates and often also 
with drugs. These aim at the cure of specific disorders, 
such as nervous debility, sleeplessness, etc. Their claims 
are often extravagant. Amongst all the multitude of 
prepared foods, there deserve particular mention the partly 
predigested foods. In cases where the digestive functions 
are weak or disordered these products have been of real 
service. 

One of the most useful and valuable of animal food 
proteins is obtained from hen eggs. The " white " of eggs 
is almost pure albumin, and there is much protein in the 
yolk also. Eggs are now produced and imported by the 
million, and form a most important item in the country's 
dietary, the protein being in a very easily digestible form. 

It is also necessary to refer to the importance of cows' 
milk as a source of animal food protein. The amount of 
protein in milk (4-5 per cent.) is not large, but it is united 
with fats, carbohydrates, salts, and vitamines in such 
proportions, that milk is about the only article which may 
reasonably present a claim of being a complete food. Milk, 
moreover, forms the staple diet of infants and young 
children, so that its protein is certainly of great importance. 
As an infant food, cows' milk is not altogether ideal. Even 
when the proportions of fat, carbohydrate, and protein have 
been adjusted to resemble human milk, there remains the 
difficulty that some of the proteins of milk (especially the 
casein) are too indigestible for young infants. This difficulty 
has been only partly surmounted by those industries engaged 
in manufacturing infant foods. Some claim to remove the 
bulk of the casein ; others to have rendered it digestible by 
treatment with enzymes ; others, again, simply claim to 
supply concentrated cows' milk. Tinned milk, generally 
concentrated to some extent, now forms a useful addition 



FOOD PROTEINS 277 

to animal food products. The casein of milk also finds 
some outlet for industrial purposes. When treated with 
formaldehyde it yields an artificial horn much used for the 
preparation of imitation tortoiseshell. Skim milk is treated 
with caustic soda or carbonate of soda, the casein pre- 
cipitated by acid, pressed, impregnated with formaldehyde, 
and dried. The product is termed " galalith." It can be 
distinguished from real tortoiseshell by the action of fuming 
nitric acid (see J. C.S.I. , 1909, 101). 

The utilization of the blood of animals, which is very 
rich in protein, as a foodstuff has long been known, but has 
met with a good deal of prejudice in this country. This 
prejudice has arisen not merely from the objection to blood 
as food, but also from the fact that such foods have been 
particularly liable to putrefaction and hence to cause 
poisoning. The shortage of all foodstuffs occasioned by 
the European War did much to overcome this prejudice, 
and there were considerable developments in the manufacture 
of black pudding and similar preparations of animal blood. 
The same circumstances made it necessary to consider more 
seriously the possibilities of other butchers' offal as human 
food, and resulted in new preparations of tinned animal 
proteins being placed on the food market. 

The author would like to record his opinion that by no 
means the last word has been said on the question of drying 
as a method for preserving animal food proteins. There is 
much to be said for this method on every ground in theory, 
and it is evidently an increasing success in practice. Dried 
milk has been followed by dried eggs, and in view of the 
success of the method when applied to fruits and vegetables, 
there seems a prospect of better success in respect of dried 
meats. After all, animal food proteins are chiefly lyophile 
colloids, and though desiccation presents some practical 
difficulties, the subsequent imbibition (assisted perhaps by 
lyotrope influences) seems to be the ideal method for 
restoring preserved protein to its original condition. 

In conclusion, it will be interesting to note in the sub- 
joined table, the relative importance of the different sources 



278 



ANIMAL PROTEINS 



of supply of both animal and vegetable food protein. The 
figures are taken from the report of a Committee of the 
Royal Society. They show the average quantities of food 
materials (imported and home produced) available for the 
United Kingdom during the five years 190 9-1 913 inclusive, 
together with the amounts of protein, fat, and carbohydrate 
present and the energy value. This information formed the 
basis of the Committee's recommendations as to economy 
of protein during the war shortage. These recommendations 
included the more economical production of meat by 
slaughtering cattle younger and the saving of 55,000 metric 
tons of protein annually by adopting cheese-making as a 
general practice in place of butter- making. 



Metric tons. 



Protein. 


Fat. 


549,000 


63,000 


356,000 


799,000 


42,000 


31,000 


91,000 


17,000 


199,000 


686,000 


9,000 


14,000 


120,000 


10,000 


5,000 


18,000 


67,000 


13,000 



Carbo- 
hydrate. 



Energy 

value, 

millions of 

calories. 



Cereals 

Meat 

Poultry, eggs, game, and^ 
rabbits . 

Fish .... 

Dairy produce, including lard \ 
and margarine 

Fruit 

Vegetables 

Sugars (including cocoa, etc.) 

Other cottage and farm pro- 
duce .... 



3,628,000 



258,000 

222,000 
1,031,000 
1,572,000 

55 1 . 000 



17,712,000 
8,890,000 

461,000 

531,000 

8,253,000 

1,077,000 
4,812,000 
6,633,000 

2,655,000 



Section IV.— MISCELLANEOUS ANIMAL 
PROTEINS 

The excreta of animals include animal proteins of great 
importance to agriculture and horticulture, forming the 
staple supplies of manure. The manure of animals should 
contain not only the solid waste material and undigested 
food, but also the urine, which contains much nitrogen, 
and hence makes considerable difference to the value of the 
product as a fertilizer. If the animals are fed on rich foods, 
the manure obtained is correspondingly richer, especially 
in its protein content. 

The value of dung manures depends not merely upon 
the protein content, but also upon its content of phosphate 
and potash, as well as other organic matter. The protein 
breaks down into simpler nitrogenous compounds, and 
eventually, through ammonium carbonate, it becomes 
nitrate. Nitrogenous manures darken leaves and increase 
growth considerably. Dung manures are deficient in 
phosphates and potash and are of value partly as nitrogenous 
manures producing growth, and partly as dressings of organic 
matter for soil. From both points of view it is desirable 
that the manure should be well decayed. Fresh dung 
manures are both wasteful and injurious to soil, except 
perhaps to very stiff clays. They are wasteful inasmuch as 
much ammonia escapes, and injurious inasmuch as they 
cause the " denitrification " of the valuable nitrates already 
in the soil. When possible dung manures should be kept 
under cover. Free exposure to air and rain will sometimes 
reduce its value by one half. It should be stored until 
" sweet," and until the straw has rotted and become 
" short." This takes usually several months. A ton of 



280 ANIMAL PROTEINS 

well-rotted farmyard manure contains very approximately 
10-12 lbs. nitrogen, about the same amount of potash, and 
about half that quantity of phosphates. It is, however, 
very variable. Horse manure is rather richer than cow 
manure, but more liable to loss on storage. Pig manure is 
intermediate between them. Sheep manure is distinctly 
richer in protein, and has therefore greater value as nitro- 
genous fertilizers. Poultry droppings are richer still, 
perhaps partly because they include the urinary products. 
When fresh they contain 18-25 lbs. nitrogen, 12-24 lbs. 
phosphate, and 6-12 lbs. potash per ton. When dried they 
have about double the value. Pigeon manure is even richer, 
and the pigeon loft scrapings have a manurial value about 
double that of dry hen manure, and eight times that of 
farmyard manure. Guano is much decayed droppings of 
sea birds on the tropical coasts of Africa and America. 
The supplies are now quite exhausted, and the market 
guanos are chiefly artificial fertilizers. 

There is one other animal protein which must be referred 
to before this volume is concluded, viz. silk. This is 
obtained from the cocoon of the " silkworm," which is 
the general name given to the larvae of certain bombycid 
moths. These larvae feed on the leaves of the mulberry, 
and when ready to pupate produce a considerable supply 
of a soft and delicate thread which is wound round about the 
larva itself. This is the raw silk, and it is unwound from 
the cocoon in a machine called the " silk-reel," and may 
then be wound into a thread. Two or more threads twisted 
together form " thrown-silk." Silk threads are also woven 
into cloth of characteristic texture and appearance. This 
protein thus forms the raw material of one of the most 
important textile industries. 

From the fish trade there is much animal protein, which 
is useless for food purposes and which, to avoid nuisance, it 
is necessary to convert promptly in fertilizers. During the 
herring season there is the disposal in this way of the 



MISCELLANEOUS ANIMAL PROTEINS 281 

heads, tails, and the guts. Many fish are incidentally 
caught which, being valueless as food, are yet useful as 
manure. After the extraction of oil from fish livers the 
residue is suitable for a similar purpose. These residues 
are steamed, dried, and ground up, forming fish manure, 
rich in nitrogen and often also in phosphate. 



REFERENCES. 

" Chemical Fertilizers and Parisiticides," S. H. Collins, M.Sc. 
" Organic Nitrogen Fertilizers," Part III., Section II., p. 105. 
" Fish Manure," p. no. 



INDEX 



Acclimatization in colloid systems, 

236 
Acid, ellagic, 29 

gallic, 29 

sulphurous, 227, 243 
Acid process for bone gelatine, 243 
Acids, for deliming, 23 

for pickling, 114 

in sour liquors, 29, 44 
Adsorption, law of, 43 

methods of clarification, 234 

nature of, 41 

of ions by gelatine, 211 
African hides, 15 
Albumins, 4, 240, 274, 277 
Algarobilla, 32 
Alum, 236, 240 
American hides, 14 
Animal excreta, 279 
Arsenic sulphide, 20 
Asiatic hides, 14 
Astringency of liquors, 44 



Bacteria in soaks, 16 

limes, 20 

bates, 24 

tan liquors, 29 
Bag leather, 86 
Band-knife splitting, 52 
Bark, hemlock, 34, 40 

mallet, 34 

mangrove, 35, 41 

mimosa, 33 

oak, 34 

pine, 34, 41 

willow, 32 
Basic dyestufis, 97 
Basils, 115 
Bating, 24, 94 
Belting leather, 65 
Blair-Campbell evaporator, 249 
Bleaching leather, 62 

glue. 241 
Block Gambier, 40 
Bloom, 29 
Boiling process for glue, 223 



Bone gelatine, 223 

manure, 273 

meal, 224 
Bones, 223 
Bookbinding leather, 104, 106, 117, 

120 
Boric acid, 23 
Bottle tannage, 103 
Box calf, 158 
Bridle leather, 71 
British hides, 8 
Brushing leather, 63 
Buck leather, 181 
Buff leather, 181 
Buffing, 52 
Burning in, 54 
Butt, 22 

Bye-products of the gelatine trade, 
272 

of the leather trades, 268 



Calcium sulphydrate, 22 
Calf skins, 76, 120, 156 
Casein, 276 
Cast glue, 257 
Catechin, 32 
Catechol tans, 32 
Caustic soda, 18, 20 
Centrifugal fan, 50 
Chamois leather, 181 
Cheeks, 268 

Chemistry of colloids, 201 
Chestnut extract, 36 
Chlorine bleach for glue, 246 
Chrome calf, 156 

goat, 163 

hide, 170 

sheep, 163 
Chrome tannage, 127-174 

finishing operations, 153 

general methods, 139 

history of, 127 

one bath, 149 

special qualities of, 136 

theory of, 129 

two-bath process, 142 



284 



INDEX 



Clarification of gelatine, 234 
Coefficient of conductivity, 253 
Colloid chemistry, 201 
Combination tannages, 191 
Concentrated foods, 275 
Condenser water, 251 
Conductivity coefficient, 253 
Continental hides, 14 
Crown leather, 178 
Cube gambier, 40 
Curing hides, drying, 13 

drysalting, 13 

freezing, 13 

salting, 12 

sterilizing, 14 
Currying, 49 
Cut glue, 257 



Decolorization of glue, 238 
Deerskins, 92, 181 
Degreasing bones, 224, 227 

leather, 115 
Deliming, 23 
Depilation, 19 
Divi-divi, 32 
Dongola leather, 191 
Drenching, 25, 95 
Dressing leather, 24, 65-92 
Drum stuffing, 63 

tanning, 63 
Drying gelatine and glue, 255 

hides, 13 

leather, 50 
Dung bates, 24 

manures, 279 

puers, 94 
Dyeing leather, 96 



Ears, 220, 268 

Eggs, 276 

Elastic fibres, 6, 272 

Ellagic acid, 29 

Enamelled leather, 123 

Enzymes, 24, 25, 94, 95 

Erodin, 94 

Evaporation, 37, 248 

Evaporators, Blair Campbell, 249 

Kestner, 249 

Yar yan, 249 
Evolution of gelatine industry, 265 

of leather industry, 194 
Extraction of gelatine and glue, 
230-233 

of grease, 115, 224, 227 

of phosphate, 224 

of tannin, 35 
Extracts of meat, 275 

of tanning material, 37-41 



Faces, 220, 268 

Fan drying gelatine, 257 

leather, 50 
Fat liquoring, 154 

tannages, 178 
Federation of Tanners, 198 
Fellmongering, 113 
Fermentation in bates and puers, 
24, 94 

in drenches, 25 

in limes, 20, 21 
Fertilizers, 269, 279 
Filter press, 238 
Finger test, 218 
Finishing chrome leather, 153 

heavy leather, 49 

light leather, 96 
Fish glue, 228 

manure, 280 
Fleshing, 22 
Flocculation, 237 
Food proteins, 274 
Foods, concentrated, 275 

dried, 277 
Formaldehyde tannage, 185 
Fractional extraction of glue, 230 



Galalith, 277 
Gallic acid, 29 
Galls, 32 
Gambier, 40 
Gelatine, bleaching, 241 

clarification of, 234 

decolorization, 234 

drying of, 255 

evaporation of, 248 

extraction of, 230 

properties of, 200 

raw material for, 220 

uses of, 260 
Glace calf, 156 

goat and sheep, 163 
Glazing, 97, 155 
Glove leather, 174 
Glue (see Gelatine) 

difference from gelatine, 241 
Goatskins, 99, 163 
Graining, 97 
Grease in bones and scutch, 224, 227 

in skins, 115 
Guano, 278 



Hair, removal of, 22 
Handlers, 47 
Hard-grain morocco, 117 
Harness leather, 71, 170 
Heavy leather, 7-92 

chrome leather, 170 



INDEX 



285 



Helvetia leather, 179 
Hemlock bark, 
Hides, American, 14 

African, 15 

Asiatic, 14 

British, 8 

Continental, 14 

dried, 13 

drysalted, 13 

fresh, 8 

frozen, 13 

salted, 12 
Hoofs, 268 
Horns, 268 
Hyaline layer, 272 
Hydrophile colloids, 240 
Hydrophobe colloids, 240 
Hydrosulphite of soda, 245 
Hypo bath, 128, 147 



Imitation box calf, 159 

glace kid, 163 
Imperial aspect of leather trade, 198 
Increase in strength of tan liquor, 44 
Incrustation, 254 

Influence of Lyotrope series, 206-209 
Intensive production, 194, 265 
Interfibrillar substance, 24 
Iron and logwood, 75, 83, 109 



Jacking leather, 51 
Japanned leather, 123 
Jelly, 203, 258 



Keratins, 4 

of epidermis, 272 
Kestner evaporator, 249 
Kid skins, 163, 174 
Kips, 8, 76, 159 



Lactic acid, 23 

Lambskins, 110, 163, 174 

Larch bark extract, 41 

Layaways, 47 

Layer, hyaline or glassy, 272 

Layers, 47 

Leaching, 35 

Leather, definition of, 27 

Legging leather, 76 

Levant grain, 109 

Lime, function of, in depilation, 20 

Liming for chrome leather, 127 

glue pieces, 220 

hides, 18 

leather, 83 

skins, 92 



Liquor, chrome 127, 129, 143-153 

lime, 18-22 

tan, 35 
Logwood, 75, 83, 109 
Lyophile colloids, 201 
Lyophobe colloids, 201 
Lyotrope series colloids, 206-209 



Machine fleshing, 23 

scudding, 23 

shaving, 52, 82 
Mallet bark, 34 
Mangrove bark extract, 41 
Mean temperature difference, 252 
Meat extracts, 275 
Mellow lime liquors, 18-21 

tan liquors, 44 
Memel butts, 83 
Milk, 276 
Mimosa bark, 33 
Miscellaneous proteins, 266, 279 

tannages, 174 
Mixed tannage of sole leather, 55 
Mordants, 96 
Morocco leather, calf, 120 

goat, 99 

seal, 106 

sheep, 110 
Motor butts, 170 
Multiple-effect evaporation, 249 
Myrabolans, 30 



Nature of chrome leather, 127 

of leather, 27 
Nett adsorption, 215 
Neutralization, 153 
Nitrogen in proteins, 1 

value of manures, 280 
Noses, 268 



Oak bark, 34 

Oakwood extract, 37 

Offal for sole leather, 63 

Oil tannage, 181 

One-bath chrome tannage, 149 

One-pit system of liming, 19 

Open-vat system of extraction, 

231 
Oxidation method of bleaching, 

245 



Paddles for washing, puering, 
dyeing, and tanning, 17, 94, 
96, 103 

Parker, on valonia, 31 



286 



INDEX 



Pelt, preparation of, 16, 92, 139 
Peroxides for bleaching, 246 
Phlobaphenes, 33 
Phosphate of lime, 223, 225, 273 
Picking band butts, 90 
Pickling foods, 275 

skins, 114 
Pigskins, 92 
Pine bark, 34 
Plumping, 19, 44 
Precipitation, 236 
Predigested foods, 276 
Preparation of pelt, 16, 92, 139 
Press leach, 35 
Principles of chrome tannage, 139 

clarification of gelatine, 234 

liming, 92 

vegetable tannage, 41 
Procter, definition of leather, 27 

glucose chrome liquor, 152 

on gelatine swelling, 217 

on pickling, 115 
Properties of chrome leather, 127 

gelatine and glue, 200 
Protective colloid, 237 
Proteins, classification, 3 

composition of, 1-3 

food, 274 

miscellaneous, 266, 279 

of dermis, 271 

of epidermis, 272 
Puering, 94 

Purification of grease, 227 
Putrid soaks, 18 
Pyrogallol tans, 28 



Qualities of chrome leather, 127 

gelatine and glue, 200 
Quebracho extract, 38 
Quercus csgelops, 30 

robur, 34 
Quick processes of evaporating, 248 

of tanning, etc., 194 



Rabbit skins, 221 

Raw material for gelatine, 220 

heavy leather, 7 

light leather, 92 
Reds, 33 

Reduction bleaching of glue, 243 
Refrigerator, 258 
Roans, 117 
Rockers, 46 
Roller leather, 118 
Rolling leather, 51 
Round of pits, 19, 46 
Rounding pelt, 22 



i> 



Salted food proteins, 275 

hides, 12 
Samming, 50 
Satin leather, 76 
Schultz chrome tannage, 128 
Scouring, 51 
Scudding, 22 
Scutch, 271 
Sealskins, 106 
Seasoning, 97 
Semi-chrome, 191 
Sharp limes, 19 
Shaving, 51 
Shearlings, 114 
Shedwork on gelatine, 257 

on leather, 50 
Sheepskins, 110, 163, 174, 181 
Short processes, 47, 194 
Silk, 280 
Skins, 92 
Skivers, 116 

Sludge from lime pits, 270 
Smoked foods, 275 
Soaking hides, 16 
Soda, 18, 20 
Sodium sulphide, 18, 20 
Sole leather, 55 
Sour tan liquors, 44 
Split fleshes, 76, 181 

hides, 86 
Splitting, 52 
Staking, 155 
Stocks, 18 
Stoning, 51 

Stove drying, 105, 109, 125 
Strap butts, 65 
Striking leather, 51 
Stuffing leather, 49, 53 
Substance, interfibrillar, 24 
Sulphide of arsenic, 20 

soda, 18, 20 
Sulphurous acid, 227, 243 
Sumach, 31 

use in dyeing, 104, 117 

use in finishing, 62, 84 

use in tanning, 102 
Suspenders, 46 
Sweating, 113 
Swelling of gelatine, 201-220 

of pelt, 19 
Syntans, 188 
Synthetic tanning materials, 187 



Tannage, alum, 174 
bag, 103 
bottle, 103 
chrome, 127, 139 
combination, 191 
drum, 47 



Sd- 



1 07 



INDEX 



287 



Tannage, fat, 178 

formalin, 185 

oil, 181 

with synthetic materials, 187 

of bag leather, 86 

of bridle leather, 71 

of belting leather, 65 

of harness leather, 71 

of bookbinding leather, 99, 
110, 120 

of morocco leather, 99, 106, 110, 
120 

of picking band leather, 90 

of sole leather, 55 

of upper leather, 76 

of roller leather, 118 
Tannage, chrome, of calf, 156 

of goat and sheep, 163 

of hides, 170 
Tannage, vegetable, heavy hides, 
55-90 

skins, 92-123 
Tanning, theory of, 41 

chrome, theory of, 129 
Tannins, catechol, 32 

classification of, 28 

properties of, 27 

pyrogallol, 28 
Three-paddle system of tanning 

skins, 103 
Three-pit system of liming, 19 
Tissue, adipose, 271 
Two-bath chrome tannage, 142 



Udders, 220 

Unhairing, 22, 23 

Upper leather, 76, 115, 120, 123 

Vacuum on condenser, 250 

pan, 248 
Valency rule, 131, 236 
Valonia, 30 

Vatting sole leather, 62 
Vegetable tannage, 41 

of hides, 55-90 

skins, 92-123 

tanning materials, 28 
Velocity effect on heat transference, 
253 

War, effect on methods, 194, 265 
on supplies, 11-13, 33, 277 

Warble fly, 10 

Waste leather, 270 

Wattle or mimosa bark, 33 

Waxed leathers, 76-86 

Weather drying, 49 

Willow bark, 32 
calf, 156 

Wood, J. T., action of puer, 94 

Wool, 110, 269 

Yar-yan evaporator, 249 

Zones of compressed water, 202-205 



PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, BECCLES, FOR 

BAILLI&RE, TINDALL AND COX 

8, HENRIETTA STREET, COVENT GARDEN, LONDON, W.C.3 



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